Yeah quote posts are in a tricky spot. We implemented both FEP-e232 as well as the _misskey_quote field (which usually gets checked alongside quoteUrl). This was intentional so we can get some immediate product adoption off the ground (with the unofficial key) and also be somewhat future-proof when the FEP gains more traction.
Here's a screenshot of the JSON associated with @csu 's post. The Link tag in there points to the post we are quoting