The hard copy of Six Sandarbhas that I have says the verse is not extant in today's Skanda purana version.
And by the way, either Madhvacharya or Jiva Goswami produce it only as one of the many arguments.
Jiva Goswami's quote seems to be borrowed from Madhvacharya's book since the verse is quoted in a similar fashion.
Jiva Goswami states in Tattva sandarbha: Text 28
atra ca sva-darśitārtha-viśeṣa-prāmāṇyāyaiva, na tu
śrīmad-bhāgavata-vākyaprāmāṇyāya, pramāṇāni śruti-purāṇādi-vacanāni
yathā-dṛṣṭam evodāharaṇīyāni | kvacit svayam adṛṣṭākarāṇi ca
tattva-vāda-gurūṇām anādhunikānāṁ
pracura-pracārita-vaiṣṇava-mata-viśeṣāṇāṁ dakṣiṇādi-deśavikhyāta-
śiṣyopaśiṣyībhūta-vijayadhvaja-vyāsatīrthādi-veda-vedārtha-vidvarāṇāṁ
śrī-madhvācārya-caraṇānāṁ bhāgavata-tātparya-bhārata-tātparyabrahma-
sūtra-bhāṣyādibhyaḥ saṅgṛhītāni tatra tad-uddhṛtā śrutiś
caturveda-śikhādyā, purāṇaṁ ca gāruḍādīnāṁ samprati
sarvatrāpracarad-rūpam aṁśādikam | saṁhitā ca mahā-saṁhitādikā tantraṁ
ca tantra-bhāgavatādikaṁ brahma-tarkādikam iti jñeyam ||28||
In order to prove the particular meaning shown by me, not in order to
prove the statement of Bhāgavatam[since Bhāgavatam is its
own proof (being equivalent of śruti)], statements from śruti and Purāṇas
are quoted in the Sandarbhas. Sometimes statements from scriptures I
have not personally seen are presented from Madhvācārya’s commentaries
on Brahma-sūtra, Bhāgavatam and Mahābhārata. Along with others in his
disciplic line like Vyāsatirtha and Vijayadhvaja, he was famous in the
South as guru of tattva-vāda, holding excellent Vaiṣṇava views well
known since ancient times, and was most learned in the Vedas and their
meaning. The śrutis that he quotes, such as Caturveda-śikhā and some
Purāṇas like Garuḍa, are now available in partial form or not
available at all. Other scriptures like Mahā-saṁhitā, Tantra-bhāgavata
and Brahma-tarka are also in this category.
Also another famous quote of Gaudiya Vaishavas also seems to be borrowed from Bhagavata Tattparya Nirnaya commentary of Madhvacharya on SB 1.1.1
gāruḍe ca –
pūrṇaḥ so’yam atiśayaḥ | artho’yaṁ brahma-sūtrāṇāṁ
bhāratārtha-vinirṇayaḥ || gāyatrī-bhāṣya-rūpo’sau
vedārtha-paribṛṁhitaḥ | purāṇānāṁ sāma-rūpaḥ sākṣād-bhagavatoditaḥ ||
dvādaśa-skandha-yukto’yaṁ śatavic-cheda-saṁyutaḥ |
grantho’ṣṭādaśa-sāhasraḥ śrīmad-bhāgavatābhidhaḥ || iti |
In Garuda purana it is said:
This work is most complete. It expresses the meaning of the
Brahma-sūtras and Mahābhārata. It is the explanation of gāyatrī and
reinforces the meaning of the Vedas. It is the Sāma Veda among the
Purāṇas and has arisen directly from the Lord. It has twelve volumes
and a hundred divisions. It has eighteen thousand verses. It is called
Śrīmad-bhāgavatam.