That (mis)information comes from an interview TheVerge held with Mr. Belfiore:
The phone is significantly feature complete, but we'll continue to polish and tweak and iterate the things that still need to work. We'll put features in right up near the end. But in general, the broad feature set is set.
source
Being
significantly feature complete, is not the same thing as the OS
being feature complete. Period. Belfiore also said they would be adding features right up near the end. At the time that was first stated, we certainly weren't anywhere close to "the end". Maybe we are now, but either way, it's still pretty much irrelevant, because Zulfiger is right.
In a nutshell, for the average consumer, the statement that the OS is feature complete
means absolutely nothing!
The features consumers care about are delivered through apps, not the OS. You can be sure that the week W10M is delivered to our existing handsets, is the same week people will be getting many new features through app updates. The overwhelming majority of consumes are oblivious to OS level feature updates. Those are typically of more interest to developers than they are to consumers.
In summary, that statement is only true if you don't care about scratching any deeper than the surface, and even then it's not really that relevant to consumers.