BIAB Mash Thickness, why is it ok?

I'm fairly new partial mash brewer, contemplating BIAB and I can't reconcile the BIAB technique compared to "traditional" 3 vessel brewing.

If "Traditional 3 Vessel Brewing", T3VB, uses 1.25-1.5 qt water per lb grain, and using more can be a pH issue or too thin for efficient enzymatic activity, then how does BIAB function correctly when you have all the mash+sparge water volume in with the grain?

Am I overthinking this stuff?

I don't think you're over thinking it at all. Yes, mashing at full volume can get you into trouble regarding pH in certain water chemistry/malt combinations. It also dilutes diastatic power, which is less likely an area of concern. For the complete novice, the easiest way to see if you're in trouble is to get the colorpHast 4-7 strips and actually test the mash pH on a pale beer style batch. If it's 5-5.5, you're probably fine using BIAB going forward.

By the way, there's no hard and fast rule that BIAB HAS to be a full volume mash. You can mash thicker and add the extra water right before you remove the bag.

>>By the way, there's no hard and fast rule that BIAB HAS to be a full volume mash. You can mash thicker and add the extra water right before you remove the bag.

Exactly.

When I BIAB brew a moderate to heavy beer, I hold back a couple of gallons and perform a dunk sparge. I pull the bag out, squeeze the life out of it, then place it in a second pot with the held back 2 gallons, mix it in, let it sit 5 minutes, then pull out, squeeze, and add the second pot to the first.