How does their system work and how effective is it?

It's built on FPGA and it's very effective.

https://www.ovh.com/ca/en/anti-ddos/

Game servers that are hosted on OVH get DDOSed daily and they drop like flies.

Myself and a lot of friends had servers killed both when renting VPS/Dedicated server or a dedicated "Game Server".

And all and all with considerably smaller botnets like the ones you rent for a few $ per hour.

If you are running a public server you learn quite quickly that if you permaban a cheater or just some annoying kid you should expect to be DDoSed these days.