Ha. Not only does the doc for syncheck:add-definition-target lack explanatory text, the bluebox signature seems wrong. It is not (syncheck:add-definition-target source-obj start finish style-name). Instead it seems to be this. Anyway, in that the (syntax-e id) is the interesting bit we want.
From https://github.com/greghendershott/racket-mode/issues/485#issuecomment-667206547: