The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com

21. And there is another one (i.e. the Lord who is different from the individual souls animating the sun, &c.), on account of the declaration of distinction.

There is, moreover, one distinct from the individual souls which animate the sun and other bodies, viz. the Lord who rules within; whose distinction (from all individual souls) is proclaimed in the following scriptural passage, 'He who dwells in the sun and within the sun, whom the sun does not know, whose body the sun is, and who rules the sun within; he is thy Self, the ruler within, the immortal' (Bri. Up. III, 7, 9). Here the expression, 'He within the sun whom the sun does not know,' clearly indicates that the Ruler within is distinct from that cognising individual soul whose body is the sun. With that Ruler within we have to identify the person within the sun, according to the tenet of the sameness of purport of all Vedânta-texts. It thus remains a settled conclusion that the passage under discussion conveys instruction about the highest Lord.