Can someone ever be accused of self-plagiarism after the course is over?
In theory, you could get in trouble for this, but the risk is very low.  Universities generally retain the authority to change grades and revoke degrees forever, at least when matters of academic dishonesty are involved.

However, nobody is likely to be looking over your old work to find this.  It probably doesn't even exist in recorded form any more.  So I see little cause to concern yourself, especially if this was a product of a misunderstanding.