It did happen. I put on 11kg of muscle in about 3 months. By about 7 months I had put on 20kg.
You almost certainly didn't gain 11 kg of muscle. You may well have gained 11 kg. Without knowing tons of specifics, its impossible to give an exact breakdown
Realistically, if you're a beginner, whos never lifted before, with amazing genetics, who did absolutely everything 110℅ right, you might have gained somewhere in the neighborhood of 6 kg of actual contractile muscle tissue. I'm not trying to take anything away from you, by saying you "only" gained 6 kg of muscle in 3 months, that's fucking great. 95℅ of guys at your gym would probably suck a dick for that kind of progress.
But that's basically the cap for how quickly a human body can physically synthesize muscle protein. It doesn't really matter how good your training/nutrition/genetics are, or how much gear you use. You just can't push past that limit, at least not by any meaningful amount.
Then you've got maybe another 2 kg of additional glycogen and associated water, maybe more if you were undereating carbs before you started to put on weight.
On top of that, if you've got a lot of inflammation from training, which is not uncommon in a beginner who's not used to working out, you might be holding another kilo of water from that.
If you're bulking, that means you're eating more. Let's say you've got an extra 500 g of stuff working its way through your GI tract.
Which leaves us with 1.5 kg fat gain
Again, these are made up numbers, but I've tried to be pretty damn charitable in my assumptions. You can rejigger them however you'd like, if you feel it would be more accurate. I'm just trying to illustrate the point that 11 kg does not mean 11 kg of muscle.
In the future, you can say "I gained 11 kg, while maintaining similar body comp", which is still really fucking impressive, while not being a transparent falsehood to anyone who isn't retarded
I also worked with a lot of guys who did use steroids, and they simply could not believe that anyone else could put on muscle without also using them.
There's a few possibilities here. Maybe they were just trying to be nice, and hype you up. Maybe you misconstrued what they said. Or maybe, they train/eat like shit, and legitimately the only reason they've ever grown is by using steroids. If its the last one, congrats on doing a better job than someone who does literally everything wrong.
Or maybe I'm wrong, and you really gained 11 kg of contractile muscle tissue in 3 months, and 9 more in the next 4 months. If that's the case, it sounds like you're about 2-3 years out from being the next Mr. Olympia. Make sure to shout out the farms when you win