Would you share some information on where this comes from. It sounds pretty authoritative. I’ve worked with people who said “I quit” and walked out. It seemed to succeed as they never came back. Could they come back the next day and say “well I didn’t specify a date therefore my resignation wasn’t valid?”
I’m not familiar with labor law, but I thought that resignation was a loose term that could fit many patterns.
This definition from feeling lucky on google [0] just says it’s the formal notice of voluntary termination and there are no laws governing resignations.
But it sounds like you have more info on this and I’d like to learn more about this.