But not with its support, because that is not the object of saṃyama.

Na means no. Sālambana is with support, with basis. Aviṣayībhūta means it is not the object.

This sutra limits the previous siddhi. The yogī perceives the pratyaya (the thought) but not necessarily its ālambana (the object upon which the thought dwells).

If someone thinks of an apple, the yogī perceives there is a visual thought, perhaps with tones of desire or memory, but does not necessarily see the specific apple.

It is a subtle but important distinction. The form of thought is known, not its complete content.

To also know the object would require an additional saṃyama on that specific object.

Patañjali is precise. He does not exaggerate the siddhis. Each power has its scope and limits.

This honesty is characteristic of the Yoga Sutras. The powers are real, but not unlimited.