I’ve seen continuous lists being interrupted by (often explanatory) paragraphs that were not part of list items. This may happen with single items, of course. Contrived example:
Fruits I like the best:
1. Apples
2. Oranges
… but cannot digest properly:
3. Bananas
So, yes, a single-item list would not have to be supported, but it can’t be distinguished reliably from a solitary item of an interrupted list.