Yes, the good thing about MPTCP is it works automatically when operating systems adopt it and add it to client OS and server OS.
They are using a proxy in the case of these smartphones because very little servers on the Internet support it right now.
It's offered as a premium service to their customers, so maybe these 5500 or so active customers have special need apps.
Operating system adoption:
iOS has support for MPTCP but it's only enabled for Siri, for testing their implementation of MPTCP I guess.
Solaris are adding it, an implementation for FreeBSD and (even multiple I believe for) Linux exist. There seems to be some customer pressure now from Linux customers to have it in mainline (financial, like Solaris). So my guess is it's going to happen this or next year ? Well I hope so anyway. The Linux best known implementation has already existed for a couple of years now.