Question: "Does this mean that today I can create a new (non existing in the native spec) method, add it in the native object, using it in a certain percentage of websites and force TC39 to change it's future implementation thus essentially hijacking the language evolution process?"
Response: "What that means is that yes, you can create a new nonexistent method, use it in a certain percentage of websites, and actively destroy any proposal trying to add that method name. I hope you don't do that, because that would be pretty sociopathic, but that's absolutely how the process works, and will continue to work for the foreseeable future.
We've made tons of progress with this constraint, and we'll continue to make progress with it."
That does not seem like a sensible constraint for language design. Just say the method name is reserved for goodness sake, you are the ONLY people who get to exercise that power.