Does this mean it should be possible to have 30 simultaneous JS8Call transmissions on a single SSB CB channel?

Yes. If you play around with JS8Call, you’ll notice that the UI picks a frequency offset from the SSB band and parks itself there. If you move the offset frequency to be near other traffic, the messages will auto-populate in the yellow text box.

Do people have to transmit after each other or can they transmit at the same time while being spaced out within the the same channel?

Data frames are synchronized to 15 second time windows (I think). You can transmit during the same window as anyone else, as long as you’re not both on the same frequency offset.