You do realize, that you only assumed the 20 seconds, right? It's not like it waits for exactly 20 seconds. But that's besides the point.
Anti KB isn't the worst hack to come across. In eggwars or skywars, they might as well start flying or use no-fall since anti KB isn't that useful.
Sure, it kicks hackers sometimes, but they'll just stop using anti KB for a bit and stick with No-fall/fly to piss others off. Why keep the kick if it's based on a hack that isn't that useful? The kick punishes legit players!
Why would you keep something useless that causes bad things?!
No i didn't assume. I got someone to personally test it using a hack client. We went into a duel together and tested. Anti-kb requires several hits to confirm the client is using this hack. And after a few hits taken, the player got kicked. Two hits with an arrow, and he got kicked.
He tried fly, depending on the settings, he either couldn't get off the ground, or if the settings were turned up, he would fly up for no more than 4 seconds before getting kicked for flying.
Obviously, different clients would probably gain different results. The hack client they used was Wurst for 1.11.
I do agree that more care should be taken to reduce the amount of false kicks, but given how difficult the task is of creating an effective anti cheat, this is obviously quite a struggle.