README says:

If the packageManager property is not present, then the fallback manager will be used; this defaults to the value of the PACKAGE_JSON_FALLBACK_MANAGER environment variable or otherwise npm. You can also provide a specific fallback manager:

It probably makes sense to check for yarn.lock etc. instead (and for the format of yarn.lock to distinguish Yarn Berry and Yarn Classic).

@G-Rath @justin808 Any objections to a PR to add this?