The cap is on MSRP. And the bill has not passed yet, so none of it is final yet.
MSRP includes factory-installed options, but not destination charge or taxes. Clearly things like upcharges for paint color, wheels, off-road package, etc. need to be part of the MSRP.
But other things purchased at the same time as the vehicle, like cross bars, Rivian charger, tent, camp kitchen etc. aren't necessarily part of the MSRP. All these things are listed in the Rivian gear store (although currently not "in stock"), so I think the intent is to allow people to purchase them later and self-install. In which case, they don't have be be included in the MSRP because they're not factory-installed options, even if they're purchased at the same time as the vehicle.
So I think there is some wiggle-room for what Rivian can put on the sticker in order to keep the MSRP as low as possible and still comply with the law. There's a lot of incentive for Rivian to keep the MSRP as low as possible, because otherwise we will all be getting rid of those high-margin accessory purchases in order to stay under the limit. They won't be selling many $5k camp kitchens since that will put most configurations over.
I think it remains to be seen exactly what the details of the tax credit will be, or whether the Senate will even pass the bill - if they don't we will get a $0 tax credit for all configurations.