I just watched "Zero"s video for the third time. I have watched other videos that he has made (dealing with induction heating) and he is a pretty smart guy. I think he is correct in that the welder, as received, IS indeed an AC output machine. I also looked up specs on it and they say "duty cycle 20% @ 90 AC amps".
It probably would be best to do a screen capture of the schematic that he shows at the beginning of his video, rather than attempting to redraw parts of it.
Then watch the video again try to follow what he SAYS, making notes as you go right on that schematic.
You WILL have to draw in a few things (that are missing from the schematic) as you go.
Another approach, depending on your comfort with electrical circuits like these, might be to start by simply making the DC rectifier change first. That one looks pretty straight forward. Then see if that works okay. Then move on to the next change, etc..
I have one of the very much older versions of that welder, so old that it doesn't have any wire speed control at all. It also has the "hot" gun, but I put up with it in the few cases where I can't get my old BetaMig 250 close enough.