Prefix MAC doesn't mean prefix the MAC. It means make a MAC by prefixing a key and concatenating that onto the data to be tagged. Basically what KMAC does: H(K+pad||message). The truncated hash functions aren't vulnerable to length extension attacks, so using SHA2-512/256 is faster than HMAC with no loss in security AFAIK.

Of course that "no loss in security" bit depends on a proper implementation. There are plenty of good library implementations of HMAC out there, and if you want a high speed MAC there's always Poly1305. But if all you have is SHA2 and don't need to interoperate with systems using HMAC, then this is a reasonably good way to go about it. It's certainly simpler than implementing HMAC on your own.