That just means that vendors should provide something that matches the specification. Usually it also means backwards compatibility guarantees, which is moot with external repos when you can stick to a revision if something breaks.
But hey, don’t worry, we’ve got std::format and soon we will have std::print. So you can keep to the standard library if you wish.
The implementation is inferior to the fmt library itself, though. An addition to fmt is a commit away - an addition to std::format is four bikesheds, three defect reports and six revisions away before it may find itself into the standard in 3 years). It has much more exposure to a much bigger and more diverse community before it hits a major version than the standard gets.
> You gave zero reasons
Just to provide the full paragraph:
> And thus far, that best tool is fmt. The syntax is cleaner, it’s more configurable without horrible side effects, the resulting operations are more efficient, and as a project it is still being actively improved over time.
Configurability: check
No horrible side effects: check
Resulting operations are more efficient: check
Under regular improvement: check
Those are my reasons. If any “pro” work negates using good tools, then I pity it. I use a range of third party libraries in my work (no, not just hobby projects - and no, not a small project) and it is a major plus.