In the presentation they said eventually kernels can share SIMT (PTX) and TileIR but not at launch. It seems pretty mysterious why they don't just emit PTX, I would guess they are either taking the opportunity to clean things up for ML tensorcore workloads or there is some HW specific features coming that they only want to enable through TileIR.

if i were to lean into cynicism, i might suggest this choice was meant to increase the effort required to reimplement cuda for other cards.