This is not correct. CMC-1 joint is often affected in longstanding RA. The first affected joint is the MP-1 joint with swelling and flexion contracture. After several years also the CMC-1 is affected with an inflammation process and thumb deformity (Nalebuff & Milender)
No, as Dr Schindele has noted, the thumb is commonly involved in RA. Attached is a link with a Table of thumb deformities in RA showing the CMC joint involvement.