No, the specification requires the former interpretation, i.e. as “quote block then paragraph” (qbtp)
cmark as C reference implementation sees it as this:
I.e. it does not see it as a continuation line.
The behavior is in an accordance with the specification:
Paragraph continuation text is text that will be parsed as part of the content of a paragraph, but does not occur at the beginning of the paragraph.
(See https://spec.commonmark.org/0.29/#block-quote, the last sentence of the paragraph about Laziness)
So, the specification explicitly says the continuation line cannot be the 1st line of the paragraph being continued from the block quote.
Also see https://spec.commonmark.org/0.29/#example-219, which confirms it.