"Do not include any repayments made later than the due date (including extensions)."

Yes. It's right there: including extensions.

"can this repayment be ear-marked for the filing of the 2020 tax return?"

The first amount, yes. That's because the tax return hasn't yet been filed, so there is the inclusion of this repayment. Otherwise, when the repayment is after the filing due date, it is part of the date you repaid it; or you would also Amend 2020.

It's all about Timing. It's an extension for the Rollover provision. It's no more or less special than that.