I just watched that video, and all he has done is disable the router by connecting the PS4 directly to the internet - which is effectively what I suggested in the first place but by a different method.
If you listen to what he says at 2:39 -
so guys, all we've done was assign a public IP address to the PS4, so it's like the modem isn't even there, it's just straight to the internet
If the router no longer has the public IP address, then the internet routing function for the LAN must be disabled, as the PS4 has become the WAN interface.
I also suspect the setup shown in the video doesn't entirely work, because his PS4 DNS settings need to be set to perform lookups from an external DNS server - rather than from the router which no longer has internet connectivity.