Synology actually has an option in their routers to "Downgrade USB 3.0 device to reduce interference of 2.4G signal" (so I assume downgrading the USB 3.0 port to USB 2.0 speeds would decrease the interference)
Yes. The interference from USB 3 connections comes due to the 5Gb/s data rate of the super speed transceivers causing interference with 2.4GHz band radio devices. If you downgrade to a USB 2.0 connection you effectively are disabling the 5Gb/s transceivers and only using the 480Mb/s data link.
I wonder if USB WiFi dongles account for this at all.
They don’t. Realtek is probably the most popular, and they’re notorious for causing problems over USB 3.