You make a good point, but the IRS will be expecting you to report this income on your 2020 tax return - if you don't include it on your return, the IRS will send you a notice proposing to increase your 2020 taxes because of this income.
The payer reported these amounts as a 2020 expense on their tax return and as 2020 income to you. You would have to prove that you didn't receive payment until 2021 and had no other options to receive payment in 2020 (e.g. direct deposit, contacted the payer regarding late payment, etc.) - and the IRS would have to accept your argument. The procedure for making your case is all manual and carried out by means of snail-mail.
The chances of success oj your part are very low - late payments on account of mail delivery are very common and not likely to make the IRS change its processing & matching procedures.
