Your strike volume is calculated with grain weight and WtG ratio.
Except for ya know, those that don't adhere to mash thickness and instead either sparge at a fixed volume, or sparge to a fixed mash volume (maxi-biab), or don't sparge at all (no sparge). There are a lot of brewers that don't care about mash thickness (wtg ratio).
At some point in the beginning of your mash, when grains have yet to absorb water, you will be in a situation where you could potentially exceed the size of your tun.
That's why you don't try to go for a mash volume of 4.99 gallons in a 5 gallon kettle. I have gotten quite close and learned the lesson. You need room to stir, my rule of thumb is at least 1" headspace above the mash.
I'm not sure you can claim equivalence on the grounds you specify. I've never seen strike water calculated using preboil volume as a variable.
Well it works just as well, and is often times a more generalized formula as mash thickness isn't fully generalized (it doesn't fit every brewer). Malted barley displaces on average 0.08 gal/lb, but that will vary slightly with how finely it's crushed and which types of malt you're dealing with. It absorbs 0.08 gal/lb for BIAB with a good squeeze. Mash tuns generally absorb 0.125 gal/lb.
If you can find any other credible source on grain displacement, let me know and I'll revise my formulas.
Here's how the generalized approach goes, if any readers use a fixed mash thickness when determining their strike volume, which I would imagine is less common for 1 gal brewers as the prevailing mashing method is no sparge BIAB (which doesn't care about mash thickness).
Strike = Preboil + MashtunLosses + Grain Absorption (0.08 for biab, 0.125 for mashtun)
or
Strike = GrainBill (lb) * Mash Thickness (or WtG for you) / 4 (qt/gal conversion)
Mash Volume = Strike + Displacement (0.08 gal/lb).