For creating reminders from a script or command-line, the usual advice is to use AppleScript and
tell application "Reminders" to do this and that—which pops the app open so that it accepts the commands. However, the ‘Fantastical’ app, at least in the first version, could create reminders without opening the Reminders app. They were still native reminders sitting there in Apple's app if I opened it. This approach has a couple advantages for me:
It's faster, since the Reminders app doesn't need to launch.
The Reminders app doesn't hang around, appearing in the app switcher and in the dock. And I don't need to close it.
So what's the trick for doing this?
I'm pretty sure there were no web APIs involved, since I never had an iCloud account and I don't think reminders can even be synced with third-parties like the calendar. Though I guess Fantastical could do something behind my back, but it's hard to imagine it happening without a trace.