They can technically just replace the key caps, but it's possible it won't resolve the issue, or the top case may suffer more significant damage during the repl

They can technically just replace the key caps, but it's possible it won't resolve the issue, or the top case may suffer more significant damage during the replacement and they don't want you to unexpectedly be without your computer for a week because the key cap repair failed. You could ask them to try it though.