Pseudo-counter point: SPM for firebase iOS SDK is in beta, and was released 2 weeks ago. Before this, you couldn't use SPM. The readme recommended (defaulted) to Cocoapods, with "experimental instructions for carthage". Therefore Cocoapods is likely used by most iOS apps with firebase.
So apparently a lot of them do support SPM even when it doesn't say so in the README. Confusing, annoying. Out of a project with 10 dependencies I thought only 4 supported SPM by reading the README but in the end only one (which was my own...) Cocoapad didn't have SPM support.