In fact they do suggest that a board with multiple designs but no V-cuts or slots you could cut it yourself, placing some guide lines on the silkscreen.
Basically fabs will charge for anything that is extra work. If they see something that looks like multiple designs or repeated single design, they will flag it in the review. V-cuts indicate panelisation, slots could. Some single designs may be grey area, like a board with a prominent slot due to the need for isolation. They may or may not invoke the extra work charge.
Not having a workshop and not willing to spend effort with a saw on FR-4, I’m happy to avoid panelisation since prices are so low already.
Jlcpcb rejected a design where I had added mouse bites and rails to a design - claiming it was two separate pcbs. I had a similar design where I had added a programming header to the rail and sneaked traces with a neckdown through the mouse bite - this design was accepted. As I understand it, they consider a continuous copper connection as a single board - but maybe you need a trace or two rather than a zone.
TBH, they are pretty cheap, often an order of magnitude less than a local supplier it’s not worthwhile trying to game their systems even more.