Longest chain does not apply in this situation since the two chains are not in a
block race competition.
What an idiot.
All forks are 'in a block race competition' each with different blocks. There is no distinction when each fork runs off with a thousand or more unique blocks or if they run off with just 3 or 4. The longest chain is Bitcoin.
No, that is semantics.
All possible fork types are not all in block race competitions, like with softforks
or hardforks. BCH was not a simple hardfork, but a "reverse bilateral hardfork"
so its chain has no connection to BTC, and thus considered an "altcoin".
If in the future BCH becomes the dominate coin for world economic purposes,
and BTC over time disappears, then BCH would be considered "Bitcoin", but not
because of the "block race" outlined in the whitepaper, but because it just took
over the cryptocurrency market in a more economic competitive way.
The whitepaper "block race" only applies when the two chains are fighting it out
to supplant the other chain, to become the only single chain. This is not occurring
with BTC and BCH, since they have been disconnected by a bilateral hardfork type.
They are on two separate disconnected chains that the Whitepaper does not ever
address. The whitepaper is written in the context of an untested 2007 thinking of
a theoretical idea, and did not plainly anticipate miners attempting to keep an
orphan chain alive through changes to the protocol and violations of consensus.
That is unaddressed since it is a violation of the "honest nodes" theory that
Satoshi incorrectly relied upon, originally.
So, block racing and longest chain does not apply here. Only who has the most
security between the two chains (ignoring other major issues that markets ignore
now, but will only be evident later in time).