This recommendation is 100% to alleviate concerns about vaccine accessibility and to calm the freak out that is building about delays in getting the second dose.
The CDC/WHO looked at the initial data and noted you can extend up to 6 weeks it’s still effective. I think 8% of the Moderna clinical study didn’t get a shot at 28 days. So we have some data ranging from early to up to 2 weeks late. I believe if you are over 4 days early the 2nd dose loses effectiveness, but it’s been a while since I looked at that data.
The FDA won’t officially support a deviation from the approved EUA. Moderna and Pfizer would need to support amendment with a sufficient data set requesting approval to change.
Moral of the story, get the second dose like you are supposed to, but if you are delayed don’t freak out or decide not to get it.
Edited to add: The CDC didn’t recommend going to 6 weeks for the 2nd dose, they said it can be given up to 6 weeks and still work.