What are you confused about?
The city hasn't gotten the funds from the Feds yet (USDOT is committed to the project but Congress has not yet appropriated the funds) so the City of Detroit is issuing bonds (generally called
GARVEEs) that will be paid back once Detroit receives the Federal grant.
Because of the time value of money and the general increase in construction costs over time, Detroit gets more benefit from paying interest on the bonds than they do by waiting a few years for the Feds to find the money. It also allows construction to start much sooner.