Is c-treeRTG's SQL full-functioning instance of c-treeRTG that supports mainframe data? Or is this a data migration tool?

c-treeRTG SQL database engine gives you many of the benefits of a full fledge relational database.
It is not a data migration tool. It runs on top of the c-treeRTG Server, which is an ISAM file system that does not run on the mainframe.

ISAM files are very prevalent with COBOL applications running on open systems (similar to the use of VSAM files with mainframe COBOL).
With c-treeRTG, open systems COBOL users can now add the flexibility of a relational database engine to ISAM files without changing current program code or data.
This eliminates the need to choose between maintaining ISAM files or moving to a mainstream relational database to meet new customer requirements for SQL access.