The PP is not appealing to authority, but making an argument against GP's claim that people are AI doomers because they are uninformed, arguing that many experts (presumably informed) are also AI doomers.

Tegmark has his name on a bunch of competently written AI papers: https://arxiv.org/search/cs?searchtype=author&query=Tegmark%...

In each case, the first author is Ziming Liu, which is more impressive than Tegmark's last author (PI) position. But still, it's fair to call Tegmark an AI expert.