You are correct to point out that the rules for government procurement are different today than in the 1980's (at least I'm almost sure they are). In any case, Cintra's contract is with the Texas DOT, not the Federal government. Setting all that aside, I was simply providing some history with regard to public/private partnerships, by way of illustrating that they're not a recently-developed concept.
So "how" is the feds giving Cintra the loans?
http://www.fhwa.dot.gov/ppp/tifiasep15.htm
Read this letter to The Federal Highway Administration applying for a federal loan from TXDOT. The read on down to the Experiment.
"Federal policies generally encourage price competition, which this concession procurement contemplates. However, we recognize that it would be burdensome for the TIFIA office to be required to work with multiple proposers during the bid preparation period, when ultimately TxDOT will award only one of them the contract"
So the state of Texas will borrow the money from the Feds and then give it to Cintra. This reminds me of "money laundering" Not kosher.