This is not a case of chicken and egg. A style guide is required to then write tooling to enforce it.
I have often seen questions similar to “where’s the style guide?” and this is always met with “there is none”. And for as long as there is no style guide, no one can follow it. When a style guide will be written up, people can start following it. Even without enforcement, this can decrease the burden of maintainership by at least having prescribed rules to follow. While not every contributors will, at least part of the core, if not all maintainers, might coalesce to a more unified style through the style guide.
I am, though, completely in agreement that enforcing the style guide without tooling is not feasible.
Lastly, it’s often been pointed out that we need nixfmt, but as it’s not been written yet, there must be reasons. Why should we stop an improvement effort because it’s not perfect? Unless you’re writing said tooling, halting efforts in their tracks is counter-productive IMO. The current status quo (no style guide, no tooling) is markedly worse than the suggestion (style guide, tooling later).