I think what he means is, from the point of the driver driving from the 50 to the 60 area:
You drive fifty, then you pass a sign pointed away from you (towards oncomming traffic) saying 50. 65m further, you pass a sign pointed at you saying 60.
In that case I would say the speedlimit is just different for that 65m stretch of road, based on the direction of travel. The speedlimit changes at the 50 sign if driving from 60 to 50, and at the 60 sign if driving from 50 to 60.
For the first issue thats not realy true. To access the totp key you still need the actual device with the key, it's only now split over multiple devices. Like having multiple bank cards for the same account.
For the seccond issue: Thats a good point, I have not found a good solution for that either, unfortunately
I use vaultwarden (selfhosted bitwarden), which stores both passwords and OTP keys on my own server, which I backup regulary. This allows acces to my OTP keys from any device, as long as it's in my local network or connected to my VPN.
Must say I really like this solution. If one of my devices fail, I have a pretymuch seamless switch to any of my other devices, which are already configured anyways, since it's also my passwordmanager.
If the server fails, my phone, pc and laptop all still have the keys cached, so I can use those untill I've restored a backup.
That might be the lense, it's easier to see if you compare the outside ceiling with the top of the elevator doorframe