Key detail you may have missed:

>you still need to pay taxes on any money in your traditional IRA that hasn’t been taxed.

So only any pre-tax money (deducted when contributed or growth after contribution) involved in the conversion incurs owing tax.

The "clean" backdoor Roth goes like this:

• You have a $0 Traditional IRA balance, so no pre-tax dollars already exist in it.
• You contribute money into a Traditional IRA. You claim this as a "non-deductible contribution" on your return to report that these are "after tax" dollars. So they get included in your taxable income on your return and taxes are owed and paid on them as part of that. There is no income limitation on this contribution.
• You convert your Traditional IRA balance into your Roth IRA. There is no income limitation on this conversion. That used to be different, but the income restriction's removal back in the 90s(?) was the change that opened up the backdoor.
• The amount converted is taxable based on a "pro rata" calculation. You take the $pre-tax / ($pre-tax + $afterTax) of your IRA balance to get a percentage. If the $pre-tax is $0 (because you didn't claim the deduction and earned no interest/growth), then 0% is taxable.

There's some wiggle room around timing of things. You don't have to wait to file your return for those contributed dollars to count as after-tax, everything gets lumped together on your tax return.