The 99 and 2000 FRC's were not Z06's. The first FRC's were equipped with the LS1 and were not much of a performance improvement over the targa coupe or the vert.
When GM says that the 63-67 were the first FRC's, that is true. The 53 through 62 were all roadsters. In 63, the Vette could be had as a coupe for the first time and the roof did not come off - hence "fixed roof coupe" - not a name they used back then, they were just "coupes". The new C6 Z06 with the fast back and fixed roof more closely resymbols the 63-67 style coupe. In 68, the targa roof was released and all Corvette coupes since then have had removal roof panel(s) until the 99 FRC.
When you say that you thought the FRC was 'removable back then', you may be thinking of the 'removable' roof for the convertible Vettes. These removable roofs were optional for the convertible up until the C5 convertible was offered. Definitely not a "fixed roof coupe" but a removable hardtop for the verts. They were notch backs which makes them look like the C5 FRC's.