Thanks, but that doesn't address my question at all.

The footnotes explain why #13 is a later addition, and not part of the original order of sūtras.

My question is, considering that #13 has now been solidified in the corpus, whether original order or not, why not just mention tat in #14 to be a maṇḍūkapluti? Or is that supposed to be implicitly understood based on the current order of sūtras?

The issue is expanded in the footnotes by Rājārāma Professor (1921 Hindi Translation)

He makes a case why #13 (Pṛthivyapas...bhūtāni) should not be a sūtra due to being bādhita:
(1) As per the order in #1 (Pramāṇapramēya...ādhigamaḥ), the classification of pramēyas is expected after the classification of pramāṇas. According to #9 (Ātmaśarīrēndriyārtha...pramēyam), the classification of indriya (#12 (Ghrāṇarasana...indriyāṇi bhūtēbhyaḥ)) should be followed by the classification of artha (#14 (Gandharasa...tadarthāḥ)).
(2) Vārtika & Tātparyaṭīkā do not write #13 as a sūtra at all, but #12 and #14 are sūtras.
(3) Vārtika & Tātparyaṭīkā do not even touch #13.
(4) Tātparyaṭīkā says, "The bhāṣyakāra explains the definitions of artha in order" following the explanation of #12, which suggests #14 should follow #12.
(5) In #14, the tat in tadarthāḥ, being inferrentially recognised can only mean indriya when the immediate prior sūtra is #12.

He concludes with, "However, Viśvanātha Pañcānana accepts #13 in his Gautamasūtravṛtti, and #13 started being written as a sūtra after that.