To me, at least, the photo looks suspiciously doctored, and I was not aware of any reason why the relatively light weight of a smartphone can cause such a deformity.
The Google Translate of the tweet information:
[Finger is deformed! ? ] ... Also depending on how to hold the smartphone to become "text thumb injury". So that it is not too much a burden on a particular finger, but please take the time to time how to hold a change or break.
Native Japanese speakers are welcome to suggest edits and/or provide a proper translation, but it seems quite clear (to a Chinese speaker) that they are suggesting that the finger deformity is caused by holding the smartphone.
The belief of "smartphone pinky" being a real phenomenon is also corroborated by Rocket News 24 (an English blog about Japan) which cites the above Twitter post, as well as Urban Dictionary, which attests that the deformation is permanent.