The key is movement. If it's all heat but straight, like Eovaldi was last summer, MLB hitters rip away. But if he throws 100 with a wrinkle, like the cutter he used as a weapon in the '18 postseason, batters have no choice but to guess.
Most unhittable pitching I've ever seen was a few years back in Baltimore, when I sat about 20 rows behind the plate on a Sale day. Fastball 100, slider 90-something, and his long sidearm lets go about halfway to the plate -- the human eye can't even see the ball until it's too late. Granted, the last-place O's were loaded with rookies, but they didn't stand a chance: 5 IP, 1H, 12K, and one hard hit foul ball that I swear was just a good guess (the pitch hit the bat's sweet spot).