I agree. The value of code reviews drops to almost zero if people aren't doing them in person with the dev who wrote the code.
Firstly, we get important benefits even when there's nothing to talk about: we get to see what the other person is working on, which stops us getting siloed or working alone. Secondly, we do leave useful feedback and often link to full articles explaining concepts, and this can be a good enough explanation for the PR author to just make the requested change. Thirdly, we escalate things to in-person discussion when appropriate, so we end up having the most valuable discussions anyway, which are around architecture, ongoing code style changes, and teaching/learning new things.
I don't understand how someone could think that async code review has almost zero value unless they worked somewhere with a culture of almost zero effort code reviews.
Sometimes a PR either merits limited input or the situation doesn't merit a thorough and thoughtful review, and in those cases a simple "lgtm" is acceptable. But I don't think that diminishes the value of thoughtful non-in-person code review.
Which is awesome and essential!
But the reason that the value of code reviews drops if they aren't done live, conducted by the person whose code is being reviewed, isn't related to the quality of the feedback. It's because a very large portion of the value of a code review is having the dev who wrote the code walk through it, explaining things, to other devs. At least half the time, that dev will encounter "aha" moments where they see something they have been blind to before, see a better way of doing things, spot discontinuities, etc. That dev has more insight into what went into the code than any other, and this is a way of leveraging that insight.
The modern form of code review, where they are done asynchronously by having reviewers just looking at the code changes themselves, is not worthless, of course. It's just not nearly as useful as the old-school method.