Of course it is true. They didn't repeal F=MA. It's just that the rearward force against the fuselage from the cannon firing is proportionally small enough that the velocity reduction isn't noticed. It is equivalent to the drag force operating on the aircraft.
It would make a good physics problem for high school students to solve. "How much does the A-10 slow down when the GAU-8 Avenger is firing at a rate of 1,800 rounds per minute?"
Instead they get asked to solve "If Heather has two mommies, and each of them has two brothers, and one of those brothers has another man for a 'roommate,' how many uncles does Heather have?"
“... how many uncles does Heather have?” - do we have to specify ‘uncles’ as separate from ‘uncles-in-law?’
Bonus points question: how many of the people mentioned in the problem are kwheer?
Post 18.
Slows abruptly but with the trigger pulled for less than a second, the reduction in airspeed is negligible.