Indeed the code does unconditionally set the min version (and thus ignores the system config), and also the max version (to 'any' if not configured).
Instead of just documenting this, I would rise the question if this is rather a bug that should be fixed? (i.e. use the system defaults if not set in the radsecproxy config, or from a code perspective, don’t call SSL_CTX_set_min_proto_version() if not configured in radsecproxy)