ISPs at scale do not use software routers. They use ASIC routers (Juniper/Arista/Cisco/etc.), for many reasons 1) features 2) capacity 3) reliability.
ASIC routers are capable of handling 100-1000x the throughput of the most over-provisioned Linux server (and that may even be an understatement). ASIC routers can also route packets with latency between 750us (0.75ms) and 10us (0.01ms!), complemented by multi-second (>GB) packet buffers.
QoS is rarely used at scale, if anything only on the access layer, because transit has become so cheap that ISPs have more bandwidth than they know what to do with. These days, if a link is congested, it's not cost saving, but instead poor network planning. QoS also has very limited benefits at >100G scale.
With that said, I feel that this article is definitely missing the full picture.