Dear Mr. Rosati,
I have the Bengali edition of the Devīpurāṇa you are referring to. The verse 39.6 fully quoted runs thus (without removing the doubled consonants used by the editor):
sarvvakāmasusiddhyarthaṃ rājati vedaparvvate |
yajed bhaumātjamjo devīṃ kāmākhye girikandare ||
The reference to the sacred site of Kāmākhyā in Assam is embedded in a longer descriptive enumeration of various sacred sites of goddesses contained within 39.2-39.21 ('brahmaṇā puṣkare devī….plavā helā prakīrtitāḥ') of the Bengali edition. I do not have the Devanāgarī edition, hence I am unable to check if this rather interesting little passage on what seem to be important Śākta pīṭhas of the time is included in it. If it is not, let me know and I can scan the passage from the Bengali edition for you.
I would interpret Bhaumātmaja as "a descendent of [king] Bhauma", in which Bhauma, according to royal genealogical myths of Kāmarūpa from the fourth to the twelfth centuries CE, seems to refer to the alternative name of Naraka, the mythical first king of Prāgjyotiṣa. The second half of the verse means accordingly: "A [--by extension, 'any'--] descendent of Bhauma [the first king of Kāmarūpa] must worship the goddess in the mountain cave Kāmākhyā". For a description of the myth of the first king of Kāmarūpa, the genealogies of that kingdom and the Kāmarūpa kings' loyal patronage of the site of Kāmākhya for the authorization of their political power, see J.E.Shin, "Changing Dynasties, Enduring Genealogy: A Critical Study on the Political Legitimation in early Mediaeval Kāmarūpa". I attach the article herewith.
I do hope this may be of some help.
With best wishes,
Bihani Sarkar BA, M.Phil, D.Phil (Oxon), British Academy Postdoctoral Research Fellow in Sanskrit
Christ Church College
Oriental Institute,
Oxford University