Again, "I had no idea it wasn't considered a solid measurement" does not mean "I am asserting it is a solid measurement". It is quite obviously not the same statement (use of the past tense, for one - they make no claim about what ideas they have now), "from the words in the exchange". It is true that sometimes "I had no idea that X" is used to mean "I don't believe you that X." It is also frequently used sarcastically to mean "obviously X". But from context, I think the statement was quite literally a statement that they had no idea. This was relevant because, again, you had just asserted to someone else that it was unreasonable to have no idea.

Lastly, charity has a role in any conversation - particularly those involving disagreement. If your conversation partner might be saying something stupid or something reasonable, taking the reasonable interpretation or checking what they meant produces better conversation. I myself would sometimes do well to remember that, in the heat of discussion.