Or rather because the trades could be done internally (Fidelity could sell off the stock they own)
EDIT: It seems my assumption was incorrect. This link posted by grandmczeb shows that RH needed more capital by a downstream dependency to ensure those trades: https://archive.is/GFtf2
They owned 13% of the company; now they own only 87 shares; they sold 9.3 million shares in 1 month.
So, during the Robinhood GME debacle, Fidelity could sidestep the issue, since they wanted to sell their own GME shares anyway, and also managed to look good in the process (no GME buying restrictions compared to Robinhood etc); win win for them
EDIT: It seems my assumption was incorrect. This link posted by grandmczeb shows that RH needed more capital by a downstream dependency to ensure those trades: https://archive.is/GFtf2