I think the wakeup_count model would require the pm-daemon, between reading wakeup_count and writing back to it, to say to the alarm timer "you aren't doing anything right now, are you" and for the alarm timer to check and say "no, I'm right - nothing happening here". Only then does pm-daemon go ahead and suspend. In your case the alarm timer would say "Oh, I just got a wake-up, better not suspend just yet".
The only place I can see room for a race is inside the kernel as the event is handed from one subsystem to another. If e.g. an interrupt handler registered a wakeup event, then queued it for a kernel thread to handle and make available to user-space. I cannot immediately see how we are guaranteed not to sleep before the event becomes visible to user-space. However I suspect a very short usleep in pm-daemon would cover the race, and there may be some rules for writing wakeup-aware drivers that I don't know about.