Don't non-mining nodes contribute towards consensus? I mean isn't that how user activated soft forks work? My understanding is that in 2017 non-mining nodes essentially voted against the majority of miners to activate the SegWit soft fork.

I think it depends on what type of security you are talking about. Obviously non-mining nodes don't contribute towards double-spend / 51% attack resistance, but they do certainly have influence over the network, in a way securing the protocol from centralized changes.

I definitely agree that multiple nodes controlled by one person doesn't help secure the network any more than running a single node.