But in the details this attack is not that bad. E.g. NordVPN and I guess also other VPNs use firewall rules to drop traffic on normal network interfaces.

Their side channel is still routing traffic away from the VPN channel. Then they can observe that there is no traffic and guess that the user either didn't make requests in that moment or that he wanted to visit a website in the range covered by the route. They can not spy on the traffic.

Also you can not quickly move into a network and apply this attack, as DHCP leases usually last 1 day or at least 1 hour. Only when they expire you can apply the attack (or you force the user to drop from the network, which is easy if they are using WPA2, but only possible by blocking the wifi signal if they are using WPA3)

It is a serious issue and should be mitigated, but not as huge as news articles make it.

I was going from this: (emphasis mine)

Interestingly, Android is the only operating system that fully immunizes VPN apps from the attack because it doesn't implement option 121. For all other OSes, there are no complete fixes. When apps run on Linux there’s a setting that minimizes the effects, but even then TunnelVision can be used to exploit a side channel that can be used to de-anonymize destination traffic and perform targeted denial-of-service attacks.

Partial context is a bitch.