I want to reply to a poster on such thread legitimately

That should be all you need to consider. A node is just a node and if you think that the reply would be a net benefit to the monastery then post it. I suppose that it's conceivable that the reply might only be of benefit to the parent author in which case you could msg them instead but such cases are probably few and far between.

I have been accused of raising posts for XP+ before, but those who raised that are irrelevant to me.

Quite right - water off a duck's back. Life's too short.