(The funny thing here is that I don’t personally have a laser or smoothie…)
The 12V supply feeding the buck converter has inductors of its own and doesn’t need that protection. The fan you show also has inductors (electromagnets are how fans work, after all). It’s very common to power them directly from 12V power supplies for fans that should always be running while a device is turned on.
The MO1 is using an opto-isolator for the PWM signal which makes sense, and then the opto-isolator switches the gate in the MOSFET in the MO1. So the “simpler” version would be… building the MO1 yourself, possibly without the opto-isolator but frankly it’s a good idea and super cheap; the opto-isolator is a “popcorn” part (I bought 50 or 100 because it was about the same price as 1, last time I ordered any).
So I’d definitely suggest that go with using the MO1 they provide rather than trying to do without.
If it’s a separate part that you want to not order, then the MO1 is the equivalent of the MOSFET boards that you can buy extremely cheaply for switching DC-powered heated beds for printers, that people sometimes use to run the heated bed off a separate (sometimes higher-voltage) supply. By way of example only and not endorsement or otherwise, this kind of thing:
If you look at the pictures there, you’ll see that it has an opto-isolater and a mosfet. In other words, it’s essentially the same thing.