[Brightcolours]

The Sony mirrorless stuff doesn´t feature afaik IBIS. Anyhow, shutter slap issues are causally not linked to presence or absence of IBIS, IBIS may contribute to it though. Why won´t the M5 have issues with shutter slap?

Because the shutter slap moves the movable sensor, which causes blur. If the camera has no IBIS, no shutter slap blur. Unless it is a Nikkor DSLR and certain Nikkor lenses with VR.