meta discussion isn't a problem. DD was wrong.
the actual problem is parochial content that isn't of general interest. that includes stuff about specific people, events, times, places, activities (including conversations) that lack objective importance and value.
say a confused idiot is arguing boring, wrong details about a sub-point of a sub-point of a sub-point, and every single sub-point is totally misconceived and he's gone way down the rabbit hole.
it's equally boring if you then reply with parochial meta ("here is where your discussion methodology went wrong when going from sub-point 2 to 3, you should have...") or parochial non-meta (specific, detailed arguments about sub-point 3 that no one cares about because no one else has the same idea you're criticizing because they don't make that exact sequence of errors to get to that bad idea).
you want to say something interesting and important that a third party who doesn't know anyone involved in the conversation could care about. it doesn't matter if this is meta (great tips on writing, on communication, on how to discuss like Paths Forward stuff, thinking methodology content, talking about methodological errors people make, talking about error-correcting methods) or non-meta (talking about parenting, dating, politics, economics, art, programming, gaming). what matters is if it's general-interest or parochial. being about one specific person or conversation is a way to make things parochial, whether it's meta (discussing the conversation directly) or not (the detailed sub-points themselves of the parochial conversation that no one cares about).
another aspect of meta discussion is it's frequently off topic. suppose originally the conversation was about schools, and now it's about discussion methods. that's a topic change. topic changes aren't a bad thing in general. conversations shouldn't be limited to the original topic. tangents should be allowed. however, topic changes can be problematic when people are disorganized which is common. disorganized people can't deal with a branching, unbounded conversation that covers many issues and deals with sub-issues, connections to other fields, etc. the problem here isn't really meta discussion, it's some people lacking the skills to deal with multi-topic conversations at all, whether the second topic is a meta-topic or not. (they'd have equal trouble talking about both school and liberalism at once, because they'd lose track of the big picture and how the two topics are connected.)
do not consider "is what i'm about to write meta discussion?"
consider "is what I'm about to write parochial? is what I'm about to write of general, objective interest to strangers?" also if you're changing the topic or adding an additional topic to the conversation, consider if you and others involved have the organizational skill to deal with it.