As you know, members can have individual BLOGS.
All of the forum posts that a member has STARTED are listed on their homepage.
You are correct: there is currently no mechanism for finding all of the posts made by an individual member - either within a SINGLE topic - or across ALL topics they have posted in. Part of the difficulty is that no method for referencing individual posts - only pages of posts. (See also: ability-to-hyperlink-to-individual-posts and forum-posting-nice-to-have-features)
Why can not chess.com have the blogs in the usual way? All posts by some person in a single topic. Much easier to follow.