Yeah I saw GitHub - syncthing/notify: File system event notification library on steroids. but got confused since what is in https://github.com/syncthing/syncthing/tree/master/vendor/github.com/syncthing/notify is not a submodule pointing to this. Since you say it is the same, I suppose this is just a copy of the repository.
At least in theory it should be possible that the kernel pushes lots of events in the inotify queue, but the user space Syncthing process does not get scheduled often enough to consume the events (because the CPU is overloaded or because the process priority is very low) until the queue is full.