Yes, I was reasoning in the context of a "hey, i'm getting back online, what's the network been up to?" scenario.
It is true that if you do not know the longest chain then you have no security. That is the foundation on which all other layers of security depend. Running a full node will not make you any more secure if you can not connect to at least one honest node. All nodes (including full nodes) must know the chaintip of the longest chain in order to sync in a trustless manner.