Is that true for most plugins ? I always assumed plugin manufacturers came up with their own algorithms instead of basically running actions.
The PhotoKit line of plug-ins are sort of unique...when we first designed them, we had a hard core of Photoshop experts who could write actions–some very, very complicated actions–that pushed Photoshop functionality to the max. The key was knowing what function to do at what stage using the right tool and blending the results back as a layer. So, people like Bruce Fraser, Martin Evening and I could write actions that could produce the specific results we wanted. So, our stuff was not a single specific exotic algorithm but a series of Photoshop functions done in the right order with the right results.
Noiseware is producing it's results from their own algorithms (although many image processing algorithms are known standards). Yes, they can do things in their own plug-in that Photoshop might not be able to duplicate exactly.
In recent years, as Photoshop's core functionality has expanded, the 3rd party market for plug-ins has diminished. Take raw processing–ACR and LR have some really good sharpening and noise reduction capability which lessens the need for 3rd party sharpening and noise reduction.
But since you are working on scans, the use of 3rd party plug-ins would offer you a lot. And don't let the attitude of your printer jade your approach...use whatever tool you need to get what you want. If that's a 3rd party plug-in instead of core Photoshop functionality, who cares? Personally, I find that sort of attitude unhelpful.