The Blockchain (proper noun) is the "best" chain (of blocks containing digital signatures) that originates from the Genesis Block (proper noun). A proper noun refers to a "unique entity." A chain of blocks that does not originate from the Genesis Block cannot be the Blockchain by definition.
Whilst true, SPV nodes have no way of validating the genesis block so they can only work when we assume the greatest difficulty chain segment is bitcoin (regardless of which genesis block the chain originates from).