That’s because marking a kernel as LTS in the manjaro-settings-manager requires an update to the manjaro-settings-manager package itself. It’s just the ad hoc way that the software was written, given that it only happens once a year that a given kernel is designated as LTS.
As explained already, the purpose of the linux-meta and linux-lts-meta packages is to make sure that your system can still boot if your EOL kernel is removed during the update process. No more, no less.
I’m afraid your understanding would be incorrect, then. Yes, Manjaro Stable is intended to be stable — and it is — but it is not a set-and-forget distribution. Please read the short essay below… 