The 2018 mini is a T2 Mac, the 2019 iMac is not. Therefore its possible they support the 2018 mini for longer.
The base 2018 mini is a dog with 128 GB storage, 8 GB RAM and no dGPU. I have one as a test machine. I wouldn’t use it for production. The 2019 iMacs 27s are far more capable with user upgradable RAM.