In my understanding, taggedPDF is a mandatory feature to prove PDF UA compliance.
Therefore, we can deduce that a PDF that is PDF UA compliant has a tagged PDF feature, but the reverse is not true; a PDF can have a tagged PDF feature without being PDF UA compliant.
Yes, being tagged and conforming to the standard at a specific level are two different things. You can't equate the feature being present to conformance.
In my understanding, taggedPDF is a mandatory feature to prove PDF UA compliance.
Therefore, we can deduce that a PDF that is PDF UA compliant has a tagged PDF feature, but the reverse is not true; a PDF can have a tagged PDF feature without being PDF UA compliant.
https://github.com/w3c/a11y-discov-vocab/blob/2c64f33aeab54a79a445bb374d25dc0efe1cf304/crosswalk/index.html#L518-L519
Related to discussion at ONIX display - other values - PDF/ UA - 1 and PDF/ UA- 2 - codes 05 & 06 #322