They were always gonna talk today about E3. As they announced for inside Xbox for today. Because they will talk about Fanfest etc :)
Right but in terms of the response, this is basically Microsoft saying "hey yup, we're still here. If you want to know about our response, we're going to be at E3 as we have said before."
It's DEFINITELY a response. They didn't just post a random reminder for no reason. It's definitely due to Sony's messaging here. Silence wouldn't be good, even if Microsoft can't/doesn't want to announce their plans right now.
It's an appropriate response, and I don't think they are hurting by letting people know when they will hear what comes next.
So yes the announcement exists before, but Sony's blog post is definitely the reason Microsoft would post a reminder about their equivalent.
Popping up at important moments is VERY important for PR to do. The competition making an announcement is one such important time. And you can do that even if you don't have an equivalent answer right this moment.