let's see... one of the many possible voicings could be (from low to high):
Fa La Re Sol Do, Sib Lab Re Fa Do, Do Sol Mi Do,
or F6/9, Bb13, C.
Considering that we could easily replace the B-flat in the bass of the second chord with an F without significantly affecting the harmonic effect, I would classify it within the scope of plagal cadences. But that wasn't the point: the discussion was about whether the Bb7 should be considered a possible substitution for the IV- (with a simple bass movement, say approximately as happens in a tritone substitution) rather than a raw modal interchange in which the Bb7 is made to originate from a hypothetical major VII.
Of course, it's a matter of hair-splitting, but in teaching I try to find the most correct principle. Generally, I present both points of view (which are complementary) to the students and let them form their own idea.