By your rational the first person should have said:

If they are running over Wi-Fi they aren't really security, they are just toys. At the very best they are visual deterrents for opportunistic people

But they didn't, they said:

If it is running over Wi-Fi it isn’t really security, it is a toy. At the very best a visual deterrent for opportunistic people

Even though they were talking about security systems which have more than one component even if only one camera.

The "visual deterrent" made the whole sentence singular. Just like when I referenced a group of locks.

Like

His baseball card collection is his most important possession.

That collection is a lot of individual things, but the group is singular.

Theres multiple locks, but we're talking about them as a group being a singular visual deterrent.

Like I said, English is a stupid language. Like how we list adjectives in a certain order, we know when it's right or wrong, but ask someone to explain why and they usually can't.