There's a third reason for silence.

I may remain silent if someone's reply makes me reconsider my earlier opinion. In that case, I'll upvote them so that they can get some visibility if they don't already have it.

Almost all of those have genuine ambiguity. The context and overall intent matters.

Say a fairly rational and reasonable contributor goes silent. They are likely just not willing to invest the time.

Say a troll drops a bomb, disturbs the convo, goes silent, and does this regularly?

The first scenario is just ordinary human dialog. The second one is a dodge.

And in many cases it means that for reasons having nothing to do with the forum, the silent person doesn't have time to log on this weekend or is too tired, distracted etc to continue the debate.