Those with at least 51% of the sustained hash power can already redefine “Bitcoin” to be whatever they want… At any time whatsoever? (assuming they stay cohesive enough as a bloc)
That statement is a bit misleading. The damage an attacker can do through a 51% attack is much more limited than that. It allows an attacker to censor transactions or perform double spends, but it does not allow them to "redefine Bitcoin" (e.g. change consensus rules, arbitrarily reassign coins, etc.).
Anyone can do that, it doesn't require 51% of the hash power. And it's already been done hundreds, if not thousands, of times (the more technical term for them is "shitcoin").
So this seems like a pointless distinction.