I don't want to get into the old battle of how to calculate duty cycle for digital modes, so won't go there. . .
But I do sometimes run my TS-590 at 100 watts on FT modes (into a resonant trap vertical) with no issues related to cooling. Before I got into digital operation, I called Kenwood tech support and asked them if the published rating of 100 watts on FM also applied to digital modes. The technician I spoke with said, "Sure. We run those rigs wide open for hours. That's what fans are for."
In actual use, my rig rarely ever runs its fans during FT operating periods. Yes, if I pound on it for many cycles over a long operating session, the fans will switch on for a couple of minutes, but then they switch off. For my normal "search and pounce" digital operation, I never hear fans at all.
So, the advice to only run the rated AM power levels may not apply universally. If a rig is rated at full power (say, 100 watts) on FM, that is a 100% duty cycle, and thus that same rig should be able to sustain similar operating conditions on digital modes without damage. Absent a similar rating for any 100% duty cycle mode, it is prudent to start at the AM rating and try higher levels, watching for unwanted behaviors such as heavy cooling fan usage.
I would imagine that if we looked at the rigs of different manufacturers in detail, we would find that each manufacturer has a different mindset as to how their rigs should be used. With respect to digital operation, that is probably similar to the common question of whether a rig should show any ALC current or not. Again, the answer is not the same for all brands of rigs. Kenwood, for example, recommends that digital users run a small amount of ALC, but definitely greater than zero. Other brands of equipment are best used with zero ALC.