Is there a possibility, that we could see AMD CPUs in these new/refreshed Macs?
No
The fact that both iMac and Mac mini get mentioned for the update is a sign that the Coffee Lake CPUs are being excised from Apple's slate. I think the CFL CPUs must be due for discontinuation within a couple of years.
The successor CPU is still in flux.
Comet Lake S runs hot compared to Coffee Lake because Intel seem to be chasing higher clock speeds as well as increased core counts to combat the threat of AMD's Ryzen CPUs. Apple would need to account for the increased heat from this generation (and subsequent ones) by accounting for it in the redesign but fitting these CPUs into an existing Mac mini without throttling could be a challenge.
Of course, Apple might use the Comet Lake S in a larger cased Mini (Cube) but by the time you've specced that up then HD630 graphics looks remarkably under specced for the money and in any case, making mini users add eGPU seems remarkably poor value for money.
Comet Lake H (as used in the 16" MacBook Pro class Mac) would allow Apple to continue using a BGA mount CPU in the iMac while keeping CPU temps down and crucially fitting into the existing Mac mini case. This could be seen as a downward step in performance but Apple could mitigate this by creating a whole different SKU (eg 24/25" iMac).
