Using external knowledge to interpret the meaning of sentences is how every communication works.

Indeed, but accusing someone of saying something based on unstated external knowledge/assumptions is the original problem here. They just needed to say words to the effect of "taken with point X what you say implies Y" and it would be fine and much less accusatory.

I don't find "it sounds like you're saying" on a rather neutral technical topic to be very accusatory, personally.

Fair point. I should perhaps have said putting words in someone's mouth. Anyway far too much on this side point, I'll bow out here.