I’m confused here, wouldn’t me underpaying on my income generate a liability that I can then claim this rebate on?

Let's make up an example. Let's say you earn $75,000/year and the tax rate is 10%. So you owe $7,500 in taxes. That is your tax liability. It doesn't matter if you have your employer deducting $144 from your weekly paycheck or $0 from your weekly paycheck.

https://apps.irs.gov/app/understandingTaxes/student/hows.jsp

You can still get a refund with this tax credit, but it has to be a refund of taxes you paid through things like your payroll tax.

Non-refundable means that if the rebate drives your owed taxes below zero you don't get the negative tax debt back.

If you don't earn much money most of your paid taxes go to SS and medicare rather than income tax, so the rebate may not do anything for you. But if you make at least median income you should be able to fully use this rebate.

If you're retired and buy one of these trucks you'd be wise to realize $100k in investment gains in that year in order to fully exploit the tax credit.