@nealio
I just programmed two 1 hour duration events (Remind me 0 minutes | Remind me never), two one day duration events (Remind me 0 minutes - all day | Remind me never - all day), and a dummy Flow to list them.

My flow output is this one:
[
{
"Subject": "Remind me 0 minutes - all day",
"reminderMinutesBeforeStart": 0,
"isReminderOn": true
},
{
"Subject": "Remind me never - all day",
"reminderMinutesBeforeStart": 720,
"isReminderOn": false
},
{
"Subject": "Remind me 0 minutes",
"reminderMinutesBeforeStart": 0,
"isReminderOn": true
},
{
"Subject": "Remind me never",
"reminderMinutesBeforeStart": 15,
"isReminderOn": false
}
]
My dumy flow design:

THis means, in order to distinguish both scenarios, you need to take into account both 'Reminder' and 'Is reminder on' attributes from each event. Please also note the 'Reminder value' when 'no reminder' is chosen is different when I programmed the event as all day (720) and when I programmed it as not all-day (0)
Please also note 'Reminder' is not null in none of these two scenarios. Actually in three of them its value is 0, in the 4th it is 720.
My suggestion is to implement the same 'dummy flow' strategy I am following to inspect what happens with whatever event type you wanna figure out
Hope this helps