Why would lowering speed of a communication make interception easier ? I am thinking in terms of Internet standards (TCP/IP, TLS, etc) and know nothing about telephony:
"This ability to downgrade users’ speed and network quality is particularly pernicious because it can not only obstruct one’s ability to use their phone, but also make whatever communication is still possible vulnerable to interception."
So, reading further, it seems to be a downgrade attack where they force the connection to be made over 2G. I wonder if that would then be visible on the victim's phone. I didn't even know 2G existed...
In the old days we had https://en.wikipedia.org/wiki/Xmodem that could send digital data over 1G sound channel. Something like that should be possible for 2G potentially adding e2ee, but it's useful for any cellular protocol, because none of them provide e2ee, not 5G, none. The article mentioning 5G as something good forgets to mention that 5G is fully inspectable.
"This ability to downgrade users’ speed and network quality is particularly pernicious because it can not only obstruct one’s ability to use their phone, but also make whatever communication is still possible vulnerable to interception."