They use BFV, which is an FHE scheme allowing a limited number of fast additions and multiplications (enough for their use case).

Zama uses TFHE, which allows any operation (eg comparisons) with unlimited depth.

So if you only need add/mul, BFV, BGV and CKKS are good options. For anything else, you better use TFHE

I was curious about that choice as well. I guess they also just wanted to operate on integers and not floats