A bit of a side-note, but calling something an "equivalent" in this kind of circumstance is a bit of a misnomer, I think. "Competitor", sure. But "equivalent" implies some sort of interchangeability to me.
What do you mean by "interchangeability"? Do they not want Cortana do all the things Siri does?
They want it to, but unless it does today, it's not an "equivalent" yet. It's splitting hairs but the choice of that word implies they're equal in a way that "Microsoft's answer to Siri" or "alternative to Siri" does not. It's an implicit assessment of its relative capability.