In this specific situation, I think the grandparent left their employer with no options after the "email that I was leaving and would not be returning for at least two weeks and pending further info about any spread". Putting this in an e-mail really forces the employer's hand. Remember that the manager does not know who is being BCCed on the conversation, and must assume the worst (that everyone the employee knows is on a BCC list). Given that state of affairs, I think any competent HR department would insist on treating the e-mail as a resignation.

Imagine someone gets up at their desk, and yells out: "I am leaving, and not coming back until you agree to my terms". That is the (assumed) situation here, and leaves management with almost no options.