I'm skeptical because the site doesn't cite any source for their info.
Furthermore, one can easily find (alas also unsourced) "alternative facts" for mortality in Spain, which aren't monotonic in the previous years, e.g. countryeconomy.com:
Additionally EuroMOMO has this graph for excess deaths for Spain (alas in a big and ever-updated page)
There's no 2020 total I can easily get from EuroMOMO, but eyeballing it, it seems to me that integrating the large spike should be noticeable in contrast to previous years, i.e. the increase in 2020 vs 2019 should be more than 2019 vs 2018 etc.
Someone in comments pointed out to Spain's own MOMO, which no doubt is where from EuroMOMO gets their data, but I don't see an obvious total there for the year 2020 either. Spain's MOMO however does offer the raw data in CSV. Adding the
defunciones_observadas field (which seems to be the best as it has a date reporting correction) for
todos (both sexes) for 2020, I get 465,535, which is a higher number than Knoema reports (by approximately 32,400 deaths). Furthermore, if do the same exercise for 2019 (data is in the same CSV), I get 394,339 deaths for 2019, which is lower than either of the other sources (Knoema ~428K or countrydata ~417K) for that pre-pandemic year. I'm not 100% sure I'm adding the right field though.
So, is that 433.13K (from Knoema) the true number of deaths in Spain in 2020? The same question goes for 2019: is 428.44K the true number of deaths in Spain in 2019?
The Knoema difference between these years is 4.69K, whereas Spain's MOMO (if I'm using the right field from their dataset and I did not mess up the addition(s)) results in a difference between 2020 and 2019 of some 71,196 deaths, an order of magnitude more than Knoema reports.