That's not at all true - at least, not for diabetics that will go into shock or experience acute symptoms.

Diabetics experience a "honeymoon" period where they will experience some symptoms - almost always enough to trigger a trip to their PCP, urgent care, ED, etc. where they will be diagnosed and put on a treatment plan.

At most they will experience one event of being "very sick" if they don't get checked out at all during the months-long honeymoon before its found.

If most diabetics didn't know they were diabetic, most diabetics would be dead.