How did he do this? Was it some sort of combination of letters, specific tabs open or something else?

You type in '10' as soon as you get in, as fast as you can. You can really type 10 whenever in Husk's fight from what I can tell but I know for a fact it works at the beginning. Keep in mind though, you're going to have to downpatch for it to work as the April Fools update that came out has the Husk Skip patched out.