There are around 3000 pools in Cardano network(they call them delegates in that link what you shared). I would expect at least 2 relays per pool, some pools have even more and I doubt that any pool run with just one relay. To take down whole network you will need to DoS(by the way currently nobody use DoS, but mostly DDoS, because DoS is attack from one location and it is ineffective, because you can just block that IP/location with FW and etc) 6000 servers in different locations and different hosting like AWS, OVH and etc. This will be defiantly not an easy task and not cheap, I can assure you. Additionally most hosting like AWS, OVH and etc has quite sophisticated anti-DDOS mechanism:
And if your pool is using other hosting it probably has some similar mechanism to prevent this.
Additionally if your pool will become target of such attacks, you can try to use hidden relays, just allow well known peers to connect to your relays and etc.
To summarize I would like to add that any service connected to internet can be target for DDoS and in theory can be taken down, even bitcoin network, but I personally think this is definitely not an issue for Cardano and in most cases if you follow best practices for system administrating, you should mitigate most of the risks.
Additionally I strongly disagree that you can easily and cheap DoS 1000 people[probably he meant servers/devices, people do not have yet IP connection, I'm still waiting for neuralink :)]