It's absolutely a legit knockdown. Problem is, knockdowns are more sort of technically defined because boxing needs to count knockdowns, whereas MMA doesn't, so it's fans are kinda loose with the rules. If a fighter has to use the cage or ropes to keep themselves standing, or if they would have fallen without the use of them, it's a knockdown.
On the night before, Loma got a 'knockdown' by the ref's call cuz Crolla was falling out of the ropes. It confused a lot of people because usually a ref stops a guy still 'standing' only to end the fight, so everyone thought it was over, but it was completely the right call. Same with Israel, he fell into the cage and use it to stand up.