As someone who consumes SF indiscriminately as an alternative to hard drugs, I can see their point, and the point they're missing. Their point is to some extent political -- Science Fiction that uses the trope of fantasy escape hatch over and over again could lead to complacency about the real threats humanity poses to its continued existence. To write about a future that is scientifically possible, and the danger in what people are actually doing and how their actions extrapolate, is probably a good thing.
On the other hand, it diminishes Science Fiction considerably to put it in the position of being a sort of Public Service Announcement. Instead of Skittles, it will be Broccoli.
And the fundamental flaw in the Mundane manifesto is that all Science Fiction is FICTION. It's not really about space aliens and interstellar travel -- it's about the only thing it can be about -- human beings, and about fiction itself. In my comparative literature classes 25 years ago Sci Fi's core technique was given the name 'estrangement' -- no matter how foreign to our everyday experience SciFi is, its fundamental purpose and effect is to comment on our real lives. How can it do otherwise, because who among us has any first hand experience of space aliens and teleportation?
The Mundanes can go ahead and write whatever they want according to their principles, but their sort of SciFi is only one part of the larger genre, and any claim they make on having greater validity than any other flavor of SF is specious.
Writing is like music -- genre and manifesto are a lot less important than good writing. Good writing can take any form, any subject, any language, and it's all an artificial construct of the human mind. It can be about what we call 'the real world,' to a greater or lesser extent, but to grade it's validity against the actual is ridiculous and beside the point.