Bitcoin is not the only valid blockchain in existence, if that's what you're implying (your comment is unclear to me). A blockchain's length (or more specifical

Bitcoin is not the only valid blockchain in existence, if that's what you're implying (your comment is unclear to me). A blockchain's length (or more specifically, total accumulated work), does not play a direct part in determining its susceptibility to 51% attacks. Furthermore, blockchains are only susceptible to cross-chain attacks like this when they use a proof of work consensus mechanism and also share the same hashing algorithm.