I interpret this blogpost, which is excellent btw, not as 'IPv6 is a badly designed protocol' but rather as 'we haven't done a good job at evolving our network design as a hole'.
The author explains that the only real issue with IPv6 itself, is that roaming between wireless routers wasn't addressed (he calls it 'mobile IP') and that it could be properly fixed by using another identifier for sessions (uuid,port instead of sourceIP,sourcePORT,destIP,destPORT). Which would be doable with QUIC over UDP.
Sad indeed, but also awe-inspiring. I believe he is still a great soul.