There are other possible explanations, e.g. AVC and HEVC are set to the same bitrate, so AVC streams lose quality, while AV1 targets HEVC's quality. Or they compare AV1 traffic to the sum of all mixed H.26x traffic. Or the rates vary in more complex ways and that's an (over)simplified summary for the purpose of the post.
Netflix developed VMAF, so they're definitely aware of the complexity of matching quality across codecs and bitrates.
There are other possible explanations, e.g. AVC and HEVC are set to the same bitrate, so AVC streams lose quality, while AV1 targets HEVC's quality. Or they compare AV1 traffic to the sum of all mixed H.26x traffic. Or the rates vary in more complex ways and that's an (over)simplified summary for the purpose of the post.
Netflix developed VMAF, so they're definitely aware of the complexity of matching quality across codecs and bitrates.