Inks, you asked to get a response from the manufacturer, you got one. If you want a direct response, go email them yourself, or ask Rin to. They don't have to, nor need to, source to 3rd parties if they have the equipment themselves. They admit its there, and show their smoothed graph. It still has the stated dip there and the reasoning given. Sibilance probably is the reason why it's there, I know lots of IEM companies that doe this dip deliberately because on-stage use requires this.... Especially with a microphone that is sibilant by nature.
In essence, they confirmed your findings. They stated their reason. This is in all ways 100% objective. You, at this point, are the one that's not willing to accept it. Email them yourself, or just deal with it... Sorry to have to be so blunt with you, please don't take it personally.