This was not allowed, I missed this when adding it.
Luckily we haven't released this yet :)

• [css-contain-3] What happens to other @rules inside @container? w3c/csswg-drafts#6827 (comment)
• https://drafts.csswg.org/css-cascade-5/#layer-ordering

Layers that are defined inside of a conditional group rule do not contribute to the layer order unless the condition is true or unless the conditional group rule can evaluate differently for different elements in the document.

Note: Since the layer order is global to the document, any layers defined inside an element-sensitive conditional group rule need to be accommodated when establishing the global layer order, regardless of the rule’s condition. Conditions that are global to the document, however (such as media and supports) can accommodate such layer rules conditionally.