Not always.
Let's say they are in the 22% bracket now. They put away so much that they are in the same bracket in retirement, only now the recent tax cuts have expired and they are now paying 25% on the withdrawals on the whole amount, both contribution and gains.
Had they instead put the money into a total stock index fund, they wouldn't get the tax credit, but as long as they held it over a year, they'd usually only pay 15% LTCGs. They'd also pay 15% on dividends along the way.
Plug numbers into a spreadsheet and see which comes out with more money.
There are a lot of assumptions here. Maybe they'll have a window in early retirement to convert some of the deferred savings at a lower rate. Maybe they are at a much income tax rate now then they will be in retirement. So deferring taxable income might be the right choice, but definitely not always.