Most PVC under sink siffons have a preformed connection point where most people connect the dishwasher's hose.

The ones I encountered had a tapered pipe point for several diameter drains. You saw off to the matching diameter of your hose.

Often you would also put on a flowback protection valve so dirty water from e.g. draining a full sink would not get into to hose of your appliance.

Same. Thankfully, the post explains it as needing to drill open the PVC drain pipe under a sink where the washer drain pipe connects, inside a pre-formed attachment point. See "Solution 5" through "Solution 6".

If your washer is draining and working, there's no latent issue to worry about.

The one mentioned at the very end: the drain pipe the washer's drain connected to had a PVC cover that needed to be drilled/punched out (since if a washer wasn't connected it'd be a hole in that drain!). If you're replacing another washer, it was probably already punched out.

Why wasn't there just a connector with a cap?

Even without a cap, gravity "should" prevent the drain leaking - the connection to the hose is some height above where it joins the U-trap.

If the drain backs up and the pressure is high enough to force water out, it might be enough to pop any screwed cap too.

It's cheaper to make it without a hole than add a separate component and screw threads on both components. After all, you will drill the hole zero or one time, as needed.

Pvc is welded/glued. I would have installed at least a hose clamp on this thing and would never rely on friction to keep it in place.

Because Ikea.