Arc is written in Scheme in the sense that it compiles into Scheme, and uses the Scheme read, numbers, etc. But Arc macros are not Scheme macros.

Scheme also has "CL" like macros that are hygienic. Scheme 48 for instance, supports explicit renaming macros and MIT has syntactic closures. However, most schemes have a library that adds your traditional defmacro style macros, CL users are likely to want, but there use is almost always discouraged.

As far as I know Arc is written on top of MzScheme. Besides its 'clean' macros - MzScheme supports CL like macros, too. But your explanation could be correct as well.