If you are running a SegWit node then you are not trusting anybody, and you are verifying that the transaction is correct (and as such you are verifying that the block containing the transaction is correct). If you are running a non-SegWit node, then you are effectively trusting that the longest chain that you've heard about has been verified by a SegWit node somewhere, since your node won't be able to tell the difference between a valid transaction spending a SegWit output and an invalid transaction spending a SegWit output.
A SegWit node will accept the longest VALID chain. If it hears about a longer chain, but that chain contains an invalid transaction, then the SegWit node will reject that longer chain.
Thank you Danny, that's exactly how I had interpreted it. So segwit although a softfork, did introduce trust into legacy nodes. Only new segwit nodes are fully validating transactions.