Most PC fans are 1-2w. Given the claims of "near silent", I think it's plenty safe to say these are not 5,000+ RPM rippers that are going to be in the 5-8W each range.
Given the M1 Ultra has a 2lb heavier heat sink just to dissipate the extra heat, not sure that’s a safe bet. There are other ways to reduce noise than just using lower RPM fans.
> There are other ways to reduce noise than just using lower RPM fans
No, not really. You can try adding sound dampening, like BeQuiet does, but it's not as effective as just having more lower RPM fans (although it does help with coil whine). But Apple has historically never used sound dampening, and this doesn't look like it changes that. With how "open" it is anyway (the entire back being just a bunch of holes), sound dampening wouldn't be all that effective.
I'm not really sure what you think the heavier heat sink has to do with either the fan RPM or the noise profile. The bigger heatsink is if anything evidence of larger, lower-RPM fans. They're using more surface area, so they can spread the air movement out over larger fan blades. Which means they don't need to use as high an RPM fan.
Massive (by weight) heatsink can be used as thermal mass. This delays the need to increase fan speed and allows spreading that increase over time. One thing that is more noticeable than fan speed is rapid fan speed changes -- avoiding those makes entire system seem quieter.