From the synopsis of True Q that you linked to:
[Q] then reveals that Amanda is a Q, shocking everyone else. When Crusher claims Amanda's birth parents were Human, Q reveals they were members of the Q Continuum who had assumed Human form.
I can only surmise that since both Amanda's parents were members of the Q Continuum that in that case there was no introduction of the human DNA that he was looking for with Janeway.
This is further evidenced by:
...the real reason for [Q's] examination was to determine if Amanda was actually a Q or some sort of hybrid that would have to be exterminated.
He is sent there to make sure that she is not a hybrid but a full "Q".
From The Q and the Grey:
He then explains that he wishes to end the war by adding Human DNA into the Continuum and having a baby, something that hasn't been done before.
The episode does end with the "solution" being Q mating with a female Q, however it is not stated that this act of procreation had "never been done before" but rather that adding human DNA had not.
As pointed out by @bitmask in the comments, there also was a difference in the erm...mating procedures for the two "Q" children. Which may lead to Q's presumption that what he was wont to do had not been done before: E.T. and Star Wars are in the same universe but I don't think E.T. and Star Trek are, so the mating ritual was unique in-universe.