Is brake modulation just pistons engaging in series rather than simultaneously?

Simply no.

Brake systems with small master cylinders and large slaves (even multiple slaves) apply equal pressure to all pistons in the system.

The piston seals each have a design "throw", where they naturally flex and don't slide.

Smaller pistons, and big pistons should both have the same seal "throw", so as the pressure from the lever goes into the caliper the pistons should all come out equally.

This is the case for small motion movement that happens at the bigger slave pistons.

If they we're smaller and doing a longer sliding action, then the seal drag would come into it at first, and the smaller pistons would likely move first.

But as soon as there was any resistance on the small piston the internal pressure would increase and overcome the slightly higher drag on the larger piston and move the larger pistons till it equalised. Once equalised they act as a single distributor of the force.

Modulation is where the leverage rate in the system controls the force applied. This is a combination of lever pull ratio for mechanical leverage, and the hydraulic piston surface area ratios (master Vs total slaves) for hydraulic leverage.

Pivot distances, angles and lever length has a big role in the mechanical leverage and so the majority of the 'modulation' feel comes from this.

Shimano servo wave is a odd take on this, as it physically moves the pivot distances during the lever stroke

Scar

Yeah, lubing the piston seals helps minimise the uneven friction during initial movement and the retraction when the system de-pressurises after the lever is released.

So can help with pad rubbing issues etc.

Auto-pad wear adjustment moves the resting point of the pistons slightly as the grip point becomes narrower.

So sometimes in multi-piston systems this needs a helping hand, by manually holding back the other pistons whilst pulling the lever to push q piston out a bit. If the seals aren't all at an even friction level.

Lubing and moving the pistons is never a bad thing.