The legal tender status of US dollars doesn't prevent any party, public or private, from accepting payment in other forms, whether currency or commodities.

It does mean that US courts, when awarding damages (even for a contract denominated in some other currency) will do so in dollars, and that a tender of payment in dollars for a debt will generally be given legal effect.

And, anyhow, if it doesn't require the state to actually hold bitcoins, it may mean the State just contracts with any of the many entities that takes payment in BTC and converts it to dollars. Its no different, in effect, than the state deciding to take VISA instead of just cash or check.

EDIT: on review, the bill requires that the plan includes contracting with a third-party payment processor.