Asking everyone to 100% adhere to the same standard code formatting guideline is like asking everyone to collaborate separately on writing a 100 page paper with the same writing style.
Hopefully everyone will write the paper in English (or same language), but different styles will be apparent. Some will write it well, others not. Some will use contractions, some will spell the words out fully (example: it's verus it is). Etc.
I think you touched on the most important points:
- It's a guideline
If you want the code to adhere to the same formatting, like a paper to be in the same writing style, it'll need editing and revising. The code will need to be cleaned up, reviewed, re-factored, etc.
I've never been in a shop where I was completely happy with another developer's coding style or formatting (at minimal because it's not exactly like mine). But I'll be content if I can read/understand it and if it's consistent. Everything else is the sugar on the syntactic sugar.
So to answer your question: somewhat important, but it's certainly not the end of the world if they don't.