Opinions, opinions. Fang could have worked on the 400 for 10 years. "Our new model is inferior to the old", said no business person ever. I still think the 272 is his technically best iem so far. But big deal. I still prefer the re0 and 262 over them all for their unique and unabashed Hifiman tuning.
Of course someone wouldn't say it's inferior. The first thing Fang says in the interview is "comfortable" then he mentions "better sound quality". So they made them smaller from all previous models in the RE-line. Personally I find them more comfortable for longer listening sessions than the previous models but every person has different earsize / shape so it isn't going to be great for everyone. He really doesn't say much about "sound quality" - it's even better to all the other models but how? If Jude would have listened to them before the interview maybe there would be an answer on why Fang thinks that way. And how do you measure sound quality?
Sound quality is typically an assessment of the accuracy, enjoyability, or intelligibility of audio output from an electronic device. Quality can be measured objectively, such as when tools are used to gauge the accuracy with which the device reproduces an original sound; or it can be measured subjectively, such as when human listeners respond to the sound or gauge its perceived similarity to another sound.
RE-400 has two important technologies (according to the interview): 1) Titanium coating 2) Powerful magnets
He says that the titanium coating "changes the 'sound signature' a lot". Does he get into any specifics on how? No.
I don't see "sound quality" and "sound signature" as the same thing. Sound signature is something that is more personal and everyone has their personal preference. This is mostly based on what music you prefer and so on imo. If I had a narrow taste in music then I would personally not recommend the RE-400 because of its sound signature. They are tuned to be more neutral to work with almost any music genre not just some. That is where I see the clearest difference.