STX Charts, page-998

Understand your point.

It is entirely conceivable that the index funds may commission a third party to do the short selling. Or a third party acts as a short seller at its own risk and sells the necessary number of shares to the index funds in an agreed process between the two parties.
The phrase "selling to itself" describes the principle mechanism and can also be used in this way.

Ultimately, the index fund acts as a guaranteed buyer of the shorted shares and can thus ensure that it promptly reflects STX in its fund in the necessary proportion. The new shorted shares are reduced over a long period of time according to the procedure described above and returned to the lending side.

This is how I interpret what we have seen so far in trading and what could be the reason for the existing high aggregated short selling volume.