When is pointing out that two people behave similarly a tu quoque?

There is a bit of a derail occurring in this thread starting at this post. Stripping away all the ponderous prose--some of which is unfortunately mine, I think the argument can be summarized in the following dialogue:

Person A: Trump voters didn't vote for Clinton do to her being untrustworthy because she did X.

Person B: Trump also did X, so why did Trump voters vote for Trump?

Person B: Tu quoque!!!

Is the bare fact of mentioning that Trump engaged in the same behavior that caused Clinton to be untrustworthy to Trump voters a tu quoque?

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