You fell for it, got sucked in Goof.  I almost posted that exact reply with some snarky note about "incoming in a minute".  Then I had to go out, and when I came back, there is was almost exactly as imagined it would be.
That particular poster has been posting facts that should be read in total isolation without any relation to the posts they're replying to.  In this case, the fact about BMI being posted in complete isolation with no relationship to anything else in the thread, the conversation occurring, or the post is was a reply to.
It's just a fact, in a post, in total isolation with no relationship to any other post.  That obese, as defined by the CDC, is a BMI over 30.  It's not a commentary or related to any other post at all in any way.
When read that way, it's much easier to just ignore.  Since it's not intended to be a part of the conversation flow anyway.  Someone is also never right or wrong and neither advancing or changing direction of a conversation if they're just throwing out random facts, since they're not really part of the conversation flow anyway.
It's the same reason someone may say "Vaccines have some risk associated with them".  Leaving out any context to what that risk is, what's the alternative risk, and definitely not trying to say one should think twice about getting a vaccine.  They're just throwing out this fact in total isolation with no context at all.  You know all the context that makes that statement sound stupid along with stating that they're emphatically not trying to suggest the unsaid context that one shouldn't get a vaccine or should do their own research to understand this fact.  It's as if the comment is unaware of the rest of the world or the implied associations that go with it.  It's just a fact, on it's own, in isolation, related to nothing else at all in the universe.
I think it's all performance art.  It's the only explanation that makes any sense.  You got sucked in this time.