There's a pretty big difference between “I quit” (a simple immediate present tense statement) and “I will work with you on setting a non-disruptive end date” (a statement of future intent to set an as-yet-unspecified end-date.)
Here's some (Texas-specific, but broadly similar considerations apply in other US jurisdictions) relevant information:
https://www.twc.texas.gov/news/efte/types_of_work_separation...
> I’m not familiar with labor law, but I thought that resignation was a loose term that could fit many patterns.
There are many patterns of resignation, and many patterns of firing, but the difference between resignation and firing has important legal effect and is frequently disputed, and thus regulators and courts have developed rules for determining which actually applies in a particular case.
...And that's before even getting into the deeper constructive dismissal issue.
