Of course reviewing PRs is work. I bill the time I spend on PRs to _their_ ticket all the time and that's how it should be.
I think what the OP just means is that just like keeping slack green and responding to people there, responding quickly to merge requests shows that "you're there" (i.e. "working"). If that's what's required to not get any dumb questions about the rest of his day, that's very smart. As in, it seems that the hours he "works for real" + helping out others on the team/in the org are enough to satisfy his boss that he's doing the work that he's being paid for.
Awesome for him I'd say.
This counts as work to me though?? Like there's something that feels like it's not real work because I'm not writing code, I'm reading it. But it's 100% real actual work; if you weren't doing it they'd have to hire someone else to do it.