[fsh]

It is his real name, and he also used his real surname in early blog posts.

[johnfn]

It is not his real name. Once he upended his life, he revealed his real name here[1]. It is not Scott Alexander.

[1]: https://www.astralcodexten.com/p/still-alive

[Xenoamorphous]

Scott Alexander are his first and middle names, and Siskind his last name, or that's what I've understood.