Notice that the commenters following the first post did not bother to put their replies inline. That hints at the problem - because the rich text formatting of the original message is a pain to work with for the responders.
There's no difference with modern clients to produce quotes between plain text and non-plain text. In summary, in the gmail client (and possibly in most of the others) it is literally the same as what I'm doing to quote you right now. No extra hurdles. If I were to speculate why people are not doing these little quotes of each other in that thread, it is because this type of quote replies are not very popular these days in email communication, but ultimately we are jumping to conclusions about why people are not quoting each other in that thread. The fact is that you perfectly able to do that with no problems. However, the real constraints come with plain text, where one is constrained to what they can post, and the boost mailing list is an outlier here. Claudio