It is. Had today the opportunity to look into the thermals of another RK3588(S) device in a passively cooled metal enclosure that will be presented on the 20th this month. While idle temperature is a bit on the high side temp difference when running most demanding CPU benchmark is only 10°C.
Even without any cooling at all thermal throttling effects are minimal with RK3588 as such a nicely designed metal enclosure will be sufficient also with GPU/VPU busy in parallel to CPU cores.
BTW: when designing such a heatsink enclosure the best formula is low own thermal mass, huge heatsink fins and sufficient spacing in between (otherwise some active airflow would be needed for good heat dissipation and a high thermal mass results in the whole thing getting hot and feeding heat back into the board).