It seems Ubuntu infra is hosted at cloud provider? All have the mechanisms to protect from these types of attacks. Is this an architecure design failure?

Which cloud provider? Unless things have changed, Canonical runs their own servers by leasing racks in data centres. Since one of their main offerings is managed Openstack, they favor running things on their in-house openstack deployment instead of using a public cloud (AWS etc).

If the DDoS is from residential proxies and high volume it becomes a real problem to shut down.