However, a node that doesn’t see these data is still a full node.
A full node is a software that performs
full verification on
everything, if it skips any part of the verification then it no longer can be considered a "full node".
Old nodes that do not receive the witness are no longer performing full verification hence they are not full nodes anymore.
and a pre-Segwit full node can still follow the correct chaintip, without the SPV/light-wallet vulnerability of being misled onto a malicious minority fork.
If PoW were the only metric then SPV clients can also
not be tricked into following a "minority fork" with less work since they are capable of validating (and some of them like Electrum already validate) proof of work and follow the chain with the most work.
Keep in mind that when it comes to "longest chain" it is only an argument when everything else is correct. For example if
all miners decided to produce 10MB blocks that would make that chain the chain with the most work but it still wouldn't be considered "bitcoin chain" because it breaks consensus rules. That's the same with SegWit, if
all the miners decided to spend a SegWit P2WPKH output without providing any signature, all
full nodes would reject that chain regardless of how long that is but the old nodes would accept it which is another proof that they no longer full nodes.