They all started out as mods to games. DotA specifically was a Warcraft 3 mod and ended up making Blizzard change their stance on such things because they lost such a massive IP to a different company. PUBG started as an Arma 2 mod and TF was a Quake mod. All the mendioned games effectively have their origins in mods for other games and likely wouldn't exist (at least in the form they are today) if that weren't for that, is what they presumably were indicating.

https://en.wikipedia.org/wiki/Psyonix

Rocket League was a sequel to Super Sonic Rocket Powered Battle Cars which was a totally new game but born from the studio building VehicleMod for Unreal Tournament.