@Yannick Further to the above, I noticed an inconsistency in the behaviour between windows that previously had open tabs and freshly opened windows (after quitting and reopening Orion). The latter do not have an X button on the only open tab. See the below screen recording for reference.
With this in mind, I can see why it makes sense to not have a zero-tab default, because this would have to be replicated upon startup too. It is regardless my personal preference to have a zero-tab default though, and I think it should be a toggle-able option in Orion's settings (as I am aware one-tab defaults are most people's expectation of their browser).