This is definitely the policy, but my experience is that it is up to crew discretion what they allow onboard.

We have had success on long haul (admittedly you have more space and potentially lower loading there) asking the crew at the gate who have found room onboard for the car seat.

We have always "planned" to gate check it, with it tagged and ready to go in case but "hoped" to bring out on board which as I say has worked for us.

Don't expect to have use of it during the journey though, but is handy for carrying the little one off the plane (ours settles/sleeps better in the seat than in a carrier so things are easier).

My advice would be to try at least as you have nothing to lose. Also it may get less knocks being gate checked than checked at the main baggage check in.
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