He's expounding on why he agrees it was appropriate for the Court to deny the petition for writ of certiorari. Unlike for an opinion, they need not provide their reasoning in part or in full (or indicate concurrence) for their vote, however they may.

"I agree with the Court’s decision not to take up this case. I write to explain why, in an appropriate case, we should consider whether the text of this increasingly important statute aligns with the current state of immunity enjoyed by Internet platforms." (pg. 13)