This is mostly just a misunderstanding in how the move to chat link works. It's not so simple as merely "lots of comments on a post". That comment chain was essentially one person leaving several messages in a row - but that's not something that triggers the link.
The link appears when two people are having a conversation with each other. So, you need exactly two people who have written at least three comments each (four on meta sites) within a four hour period. If a third (or fourth) person joins the discussion - then it won't trigger.
That doesn't mean that any additional person on the chat thread will prevent it, only in that period of six messages.
So this would trigger it (if it all happened in four hours):
Person 1, Person 2, P1, P1, P2, P2, P1
But this wouldn't:
Person 1, Person 2, P1, P1, P2, Person 3, P2, P1
What happened in this case was this:
P1, P2, P3, P4, P3 x6, P2 x 2 (five hours later) P2
So there's no uninterrupted chain of three comments each from two people within a four hour window.
One person monologuing in the comments won't cause the suggestion to be moved to chat.