Is a slight wave in fiber cement siding ever fixable?

That wavy look in raking light is the studs talking, not the siding. Hardie is stiff enough to bridge small dips, but when a stud bows half an inch over its length, the boards follow it and you get exactly the ripple you're describing. The crew isn't lying to you about old framing, but there's a difference between 'we told you it might happen' and 'this is just how it is.' It's fixable, and you should push on it. The question you want answered before anything else: did they strip the wall down, and did they do anything to the framing when it was open? If the studs were exposed and they just sheathed right over known bows without furring or planing them down, that's on them. This is the moment to have that conversation, while the crew is still on the job and memories are fresh.

As for the fix itself, it doesn't require tearing everything off. They can pull the courses in the wavy sections, shim out the low spots with tapered shims or build the flat spots down, and rehang the same boards if they weren't damaged in removal. Where I've seen it done well, the contractor snapped a straight line down the wall and worked to that. It's slow, fussy work, which is why crews skip it, but on a wall facing your driveway it's the difference between loving the job and resenting it.

Realistically, expect them to claim it's scope creep and quote you for the rework, or eat it if they never flagged the framing issue in writing. Check your contract. If there's a line about existing conditions or a note about bowed studs, they've got a leg to stand on. If there's nothing in writing, you've got a fair arguement that flat walls were part of what you paid for. Where's the line? Slight undulation you only catch at certain angles in low sun is normal on a resided older house. Ripples you notice pulling in at any time of day, in multiple spots, is not something you should accept on a premium job. Take photos now while the light is doing you a favor, before you make the call.

On my place we went a diferent route with the furring. Stripped it to the studs, then strapped the whole wall with 1x3s on 16 inch centers and screwed them in tight, planing down the two worst bows first. Siding went on the strapping and that wall reads dead flat even at sunset. Adds a bit of thickness at the corners and around trim, so plan for that. Might be worth asking your contractor if strapping the two bad sections only is an option, since pulling a couple courses and screwing a strip behind them is less surgical than chasing every dip with shims.

Before you have that conversation, put a long straightedge or a taut string across the wavy spots. On my garage I was convinced the studs were the problem, but the bump turned out to be one sheet of sheathing that had come loose and was bulging out. Two screws pulled it back flush and the siding sat flat again, no tear-off. A broad gentle dip running down the wall points to framing, but a single localized bump can be sheathing or a bad fastener, and that's a ten minute fix. Worth checking so you know what you're actually asking them to redo.

One thing I'd check before pinning it all on the framing is how the fasteners went in. When I sided my shed I set a few nails too deep on one course and that course alone read as a shallow dip whenever the sun came in low, because the panel gets cinched in tight right there while its neighbors sit looser. If your ripple lines up with one board or course rather than wandering down across several, that points to nailing rather than bowed studs, and it's a much smaller and cheaper fix since they can back out and reset just the offending course instead of rebuilding the wall.

Don't make the final payment till this is sorted. We had a similar sitch on our side wall years back and once the check cleared, suddenly every fix was an upcharge. I'd also walk the wall at a few different times of day before the meeting with the contractor. Ours looked way worse at noon than at dusk, and knowing exactly when it's most visible helped me explain what I wanted fixed.

Update: took the string line advice and ran it across the worst ripple. Turned out to be exactly what one of you described, a loose section of sheathing bulging out behind the boards. Pulled a couple of those boards, snugged the sheathing back flush, and the wave is gone. Same light, same angle, wall reads flat now. Glad I checked before calling anyone about reframing.