I believe that Poetry does conform to PEP 518 (i.e. it specifies `[build-system]requires/build-backend`), but not to the `dependencies` part of PEP 621 [1]. There are plans for this in the future though [2]. Though I would defer to your expertise if I'm mistaken.

[1] https://peps.python.org/pep-0621/

[2] https://github.com/python-poetry/roadmap/issues/3