MVC 6's name is now also ASP.NET Core MVC 1.x.x. It's really a version number reset. The confusing thing is that it's not plugged through everywhere and people interchange the two regularly.
#wellactually We don't talk about ASP.NET MVC as a separate thing anymore. It's just ASP.NET Core. So you might build an MVC app on ASP.NET Core, or build APIs on ASP.NET Core, etc., but it's all just ASP.NET Core.
Personally, I like the new "one true name". However, most .NET developers know what ASP.NET MVC is for, but are less clear about what ASP.NET Core is about. Hopefully this will change over time but, for now at least, MVC is still a useful term from a marketing perspective, or at least that's what my publishers tell me. :)
I actually used that version in my recent book. You can see it in this section [0] of the first chapter (which is free to read, with no sign-up) that tries to explain the confusing naming.
That tweet dates from 2010 and the same quote is on this page [0] dated 2009. Of course, it's possible Martin updated it after the initial publication. I'm not saying you copied it either, it could have been independently created.