Where do you see behavior mixing with presentation in wxnim's GenUI? The only thing that the macro is used for, besides saying what to draw and how, is to define events that the elements emit.
This approach lets you blur the lines between MVC responsibilities. If you wanted to create a widget that had some traditionally 'business-logic' role (like network sync or database interaction) you could do that. I'm not recommending that but there's value in flexibility.
The FB/google share buttons on websites totally break MVC. It's just a button but it has cookie & network traffic.
I hate those buttons, people often argue they 'break the web' because of weird privacy/performance side effects, but developers use them because it's convenient to 'drop in' the platform integrations.
By comparison -- integrating an android app with GMS requires modifying a bunch of files (xml layouts, java source, multiple build dependency files). You can't just 'drop it in'.