It's kind of right.
According to other sources the Aneyoshi monument is not a common "tsunami hazard" sign. Instead, it prohibits building homes below the marked point, and, apparently, after the stone was erected (after another, earlier tsunami; different sources mention different years), survivors moved the location of the village to a higher ground, and "the village credits that with saving the village from another tsunami in 1960". Looking it up on Google Maps, the village indeed is situated higher than the monument.
And the village did survive the 2011 tsunami relatively intact. Not all inhabitants survived, though:
The mother, Mihoko Aneishi, 36, had rushed to take her children out of
school right after the earthquake. Then she made the fatal mistake of
driving back through low-lying areas just as the tsunami hit.
A significant hit to the village's population of 34.
So as to your questions:
- not everyone survived, but the village itself was untouched - "while Aneyoshi Bay had the highest runup (38.9 m) in the 2011 tsunami, the stone was about 10 m above the inundation zone, and no houses in Aneyoshi Village
- technically yes, but it happended decades before 2011.
- apparently, the Aneyoshi marker is unique in that it prohibits building below it; usually, such markers simply denote safe zones for evacuation. That said, there were ~4000 fatalities in the Iwate prefecture during the tsunami, and this particular village was untouched - the only fatalities among the residents happened when they were outside it.
An important distinction would be that modern inhabitants of the village were kept aware of the warnings recorded on the marker. This study mentions another village in the affected area, Murohama, which was located in the tsunami strike zone. The tsunami warning tower there was destroyed in the earthquake, but the villagers still evacuated to a higher ground, and not to the closest hill, which houses another stone with an inscription recounting how villagers hiding on this hill were caught in a tsunami because the hill was too low. The study notes that these were the only locales where the stone warnings entered the folklore and were commonly known, and compares them to other cases where similar markers were forgotten:
For instance, after the earthquake people in Kesennuma, a village
close to Aneyoshi, returned to their houses in the low-lying areas,
although messages on stone markers warned: “If earthquake comes,
beware of tsunami” and “choose life over your possessions and
valuables” (Macan-Markar, 2013).
This article offers an interesting hypothesis: the markers in Aneyoshi and Murohama are specific, offering practical advice such as "don't build here, build higher" and "don't go to this hill to hide from tsunami, find a higher place". They are also linked in public consciousness to specific incidents from which these warnings were borne. Most of the stones have more abstract warnings, such as "beware of tsunami", which are easily forgotten. Disaster-prevention training focused more on modern warning systems, which in many areas were rendered inoperable, and the infrastructure was designed for more common tsunamis, with breakwalls that were too low and evacuation centers in several cases located in the zones hit by the one-in-a-thousand-years tsunami. The article mentions Yanosuke Hirai, a Japanese engineer known for his insistence on the importance of taking highly precautionary protective measures against earthquakes and tsunamis. He is said to have often made reference to the great Jogan tsunami of 869, which reportedly travelled 7 kilometers inland from the coast and even reached the Sengan Shinto shrine in Iwanuma, his birth town. Hirai took part in the design of Onagawa Nuclear Power Plant and is recorded as being the only member of the project team who had insisted on making the plant's tsunami breakwall 14.8 meters tall; others considered 12 meters to be sufficient. Onagawa NPP was twice as close to the epicenter of the 2011 earthquake as the infamous Fukushima Plant, but managed to survive both the earthquake and the following tsunami without any significant damage.
- "Preservation of Records, Knowledge and Memory across Generations (RK&M).
Markers - Reflections on Intergenerational Warnings in the Form of Japanese Tsunami Stones" - OECD Nuclear Energy Agency, 2014 (pdf)
- "Learning from Earthquakes. The Japan Tohoku Tsunami of March 11, 2011" - EERI Special Earthquake Report — November 2011 (pdf)
- Nakahara S., Ichikawa M, "Mortality in the 2011 Tsunami in Japan" - Journal of Epidemiology, 2013; 23(1): pp70–73 (link)
- Fackler M., “Tsunami Warnings, Written in Stone.” - The New York Times, 21 Apr. 2011 (link)
- “The mystic stone at tsunami tide's highest point that saved tiny Japanese village from the deadly wave” - Daily Mail Online, 21 Apr. 2011 (link)
- “We’re designed to make bad choices but here’s what we can do” - The Economist, 6 Dec. 2019 (link)