Isn't this more a lesson of keeping your cards close to your chest?
There's a very good reason why people are on guard with pitch decks. Cause those very people you pitched could just take the idea and throw money at it.
No, almost certainly the other people didn't start the project because they saw his video and were so inspired by the idea they rushed off to build it. Almost certainly, it was a case of parallel evolution where they faced a similar problem and there are only so many parameters in the design space so they figured out a similar solution.
People who focus on ideas think their ideas are so inspirational and unique that they must be protected. In reality, anyone who has tried to go out actively and convince anyone else to build their "brilliant" idea quickly realizes the level of indifference and lack of engagement anyone else has to your baby.
That "those guys" may have seen the video he posted was just an unsubstantiated suspicion of his though. The sad part about having brilliant ideas for apps (as I have also found out, fortunately before putting a lot of effort into the idea) is that there are millions of other developers in the world, and one of them is almost guaranteed to have had the same idea a bit earlier than you. Unless it's a super niche thing, but then the market for it is also super tiny...
After years of building in public, I'm scared that you're right. But the problem is, my app is now live (benji.so), and it's publicly out there for people with VC money to start copying features from it ...
There's a very good reason why people are on guard with pitch decks. Cause those very people you pitched could just take the idea and throw money at it.