If you check the source of the website that it links to [1], on line 168, we have this

<p>I'm on Mastodon as <a rel="me" href="https://hachyderm.io/@simontatham">@simontatham@hachyderm.io</a>.</p>

If you trust that website, then you can be sure that this Mastodon account is the right one.

1. https://www.chiark.greenend.org.uk/~sgtatham/

Sure, but by the time you've verified that, you could also have just visited the PuTTY website (the old/current one) to verify that putty.software is legit.