That account was created in October 2016 and deleted by its owner in 2019. Since we don't keep personal data of deleted accounts I cannot tell you who it was but I am 99.999% sure it wasn't actually Donald Trump. Both deleted and suspended accounts return HTTP 410 Gone. I don't know why this is getting posted now as news.
That being said, the actual Donald Trump would not be welcome on mastodon.social, nor would he be welcome on most servers that I am familiar with. He would likely have to host his own Mastodon server.