The questions and answers are for the TY2021 preparation. If the taxpayer is asking "what can I do if someone else claimed me", I believe the part you might be missing is "if someone claimed me on their 2021 tax return"

In that scenario, the taxpayer cannot claim themselves and would not be eligible for the RRC on their  2021 return.

If they are not a dependent on a 2021 tax return and they did not receive the 3rd EIP, they are eligible for the RRC on their 2021 tax return.