The claim: once budgets are set correctly (see Daily Budget Formula (Target Units × Price × Target ACoS), New-Launch Ad Budget Formula (CPC ÷ CVR × Target Sales)), they stop being the thing that actually controls spend — bid management does. A well-run account shouldn't be routinely hitting its daily budget cap at all; if bids are set and adjusted correctly (see PPC Starting Bid Formula (Target ACoS × Price × Conversion Rate)), spend self-limits before the budget ceiling matters. See Time-in-Budget Diagnostic (Budget vs. Bid Adjustment Decision) for the metric that reveals whether budget or bid is the actual constraint on a given campaign.
Some sellers apply this literally: they set budgets deliberately far above what could plausibly be spent, so the cap is never the binding constraint, and control spend entirely through bids and bid adjusters.
This doesn't make the budgeting math upstream pointless — going through it is still how you find out what should be spent per day, whether or not a hard daily cap ends up doing any of the enforcing.