The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com

47. But (the agnis rather constitute) a vidyâ, on account of the assertion (made by the text).

The word 'but' sets aside the pûrvapaksha.--The agnis built of mind and so on are to be viewed not as complementary to a sacrificial action, but as independent and constituting a vidyâ of their own. For the text expressly asserts that 'they are built of knowledge (vidyâ) only,' and that 'by knowledge they are built for him who thus knows.'