Yeah, though you might sort of be correct, but not always. So it's hard to say if you're "TA". I have six-fig assets, and obviously not buying apple-glasses, but if I were I would have to "save up for it". Because my cashflow is set up so anything extra after standard expenses goes to investing. So no, I don't regularly have cash in checking account for $3500 toys. And I wouldn't want to sell VTSAX to fund it. I'd rather divert future investing cash towards it (just typing that makes me cringe, lol. But point still stands, if that's what one wanted to do).
Personally I probably have a too-large EF compared to most here, $10-20k, so something minor like $3.5k I'd just take out of savings and refill, because I'm confident I have the self- control to actually do that. But pretty sure most personal finance advisors recommend not to do that. So most people should "save up" for 3.5k purchase, either to not touch EF, and/or because everything else gets invested.
We do the same. After discovering MMM about a decade ago, I tracked spending for a while and then settled on a "number" that we need every month to pay standard expenses. That number is directly deposited into our checking account and all of the rest is auto-deducted into three other accounts (401k, HSA, high-yield savings account/EF). That number hasn't really changed even though our expenses have increased over the years, so there's almost no wiggle room, although we can cash flow an unexpected car repair, etc. So I agree with the bolded.
Our savings and EF are the same account - the first 20k is EF, everything after that is savings (goes to investments mostly, or home improvements, large unexpected expense, travel, etc). I personally thinking 20k is low for an EF by MMM standards, I usually see higher balances on the forum. But we have access to a crazy high line of credit card credit, HELOC, roth IRAs, HSA, and additional home equity if truly needed. So I don't see the point in a larger EF.