He took it a step further then TLJ Luke closing himself off; he was able to mask himself within the "profane", and create an entirely false presence in the Force for the Jedi to sense. To them he was just an unassuming, sympathetic, but somewhat power hungry Senator. It was such a flawless cover that some Jedi even admired and actively supported himExactly but that's my point, if someone is closed to the force they are not detectable.
Palpatine didn't just close himself but he was skilled enough to consciously do it, so it doesn't matter how many Jedi were walking around - he was undetectable.