Perhaps the difference is in that every mail client I’ve ever usedEr, really? Generally they give you an option to turn on or offthreaded display. And that in itself is a huge advantage: you canchange the setting at will, depending on your preference. Often you caneven do so on a per-folder or per-account basis (at least withThunderbird you do).
presents mailing list threads (or any thread) as a singular flat stream
GMail’s webmail and Mail.app are really the only two mail clients I’ve used in the past decade or so to be honest. I think I used thunderbird for a few weeks when I was a teenager but I honestly don’t even remember anything about it (and I barely got any email back then so I think I just used the default).
TBH I find threaded views nonsensical in every medium I’ve ever seen them in, even things like Reddit or the such feel really poor to me. Either the threading is so in your face that the same points end up getting repeated at different sub threads, or the threading is so minimal that people are replying to things cross sub-thread and treating it like a “flat” discussion.