nothing about using a USB-C cable inherently means it has to support USB3.

framing it as "limits it to USB 3 Speeds" is misleading. iPhone has only ever supported USB 2, all they're doing here is continuing to not upgrade to USB 3. the meeting where somebody proposed it went like this:

hey, should we put a USB 3 chip in the new iPhone?
nah, let's just keep using the same one as the last generation