Generally the criticism (if that is how you wish to take it) I see is not that it is "just" a sport, but simply pointing out that that training for a combat sport is not the same as training for SD, which is true. Yes, some of the techniques will still work (a good punch is always a good punch) but if you want to train for SD then you train SD not combat sport, if you want to train for combat sport then you train combat sport not SD.So there you have it, MMA works against multiple armed attackers. We can lay that argument to rest next time someone criticizes MMA as "just a sport".
The problem is that some MMA fans misread "MMA is not the same as SD" as "MMA is **** and doesn't work for SD", which is not what people say, generally. Will some elemnets of MMA work for SD, obviously yes, but not always (just ask Kuae Mena & Maiquel Falco), in exactly the same way that some elements of insert name of art here will work for SD and some won't.