This is not FUD, regulatory compliance requires that devices which make unlicensed use of radio spectrum (such as Wi-Fi) must contain non-user modifiable software for their radios in order to comply with regulations, and the MT7976C chip used in this device is no exception. While most of the mt76 driver is open source, it does contain a binary blob containing proprietary firmware which must be uploaded to the chip in order for it to function.
The claim that all of the software in the OpenWRT One is open source and copyleft is therefore false, it cannot be any other way, FCC regulatory compliance and being fully open source are mutually exclusive by definition.