MyCroft isn't a Microsoft product. Tay didn't use the MyCroft AI, as they had entirely different purposes. The MyCroft AI, from what I was reading, is for the MyCroft household product (mostly), and not intended for a chatbot AI. (If I'm wrong, correct me)
It seems that Tay was based on something similar to Xiaoice, another Microsoft project.
Of course, it could theoretically be possible to actually turn the MyCroft core into a chatbot. Since it is extensible, and open source, I could see someone using the current software (with the inflection parser, especially), to make a chatbot. However, this probably wouldn't be a text-to-text chatbot, but a voice-driven chatbot. It'd be very interesting if someone does actually create this, and I'd be very willing to try it out.