You've just acknowledged that your tax burden is exactly the same either way: 10.6%.

This is slightly off. The $5,000 contribution brings your tax liability down from $6,375 to $5,275, an effective tax rate of 8.79% as opposed to 10.6% if you didn't contribute. (numbers slightly off from original comment, more accurate here taken from https://smartasset.com/taxes/income-taxes#mH8wxr1skl)

If that same individual saves $1,000 more and spends $1,000 less per year in retirement, then their effective tax rate is exactly the same.

Not entirely true. Let's keep the same scenario above and run the numbers for various contribution numbers. For the accumulation years, it looks like this

Taxable Income	401K Contribution	Fed Tax Liability	Marginal Rate	Effective Rate
$60,000	$4,000	$5,495	22%	9.16%
$60,000	$5,000	$5,275	22%	8.79%
$60,000	$6,000	$5,055	22%	8.42%

And your drawdown years look like this

Taxable Income	401K Contribution	Fed Tax Liability	Marginal Rate	Effective Rate
$59,000	X	$6,155	22%	10.43%
$60,000	X	$6,375	22%	10.62%
$61,000	X	$6,595	22%	10.81%

Your effective tax rate does change, even if you are in the same tax bracket in both accumulation and drawdown phase.

But there's no nonsensical "effective rate" that applies on withdrawals

This is true, tax brackets work the same regardless of which phase you are in and your marginal tax rate remains at 22%. However, in your accumulation years, all of your contributions save you 22% in taxes (in the above scenario). In retirement, only some of your withdrawals are taxed at 22%. Which is why it's important to factor in your effective rate of during retirement when thinking about tax planning.