The case you cite deals specifically with traffic law and can't be generalized to all situations, but let's look at it anyway.
That a Federal employee is not immune from arrest for noncompliance with State traffic regulation where performance of his duties did not necessitate such noncompliance is well illustrated by the following excerpt from the opinion of the court in Oklahoma v. Willingham, 143 F.Supp. 445 (E.D.Okla., 1956, (p. 448):
The State of Oklahoma has not only the right hut the responsibility to regulate travel upon its highways. The power of the state to regulate such travel has not been surrendered to the Federal Government. An employee of the Federal Government must obey the traffic laws of the state although he may be traveling in the ordinary course of his employment. No law of the United States authorizes a rural mail carrier, while engaged in delivering mail on his route, to violate the provisions of the state those who use the highways.
Guilt or innocence is not involved, but there is involved a question of whether or not the prosecution is based on an official act of the defendant. There is nothing official about how or when the defendant re-entered the lane of traffic on the highway. There is no official connection between the acts complained of and the official duties of the mail carrier. The mere fact that the defendant was on duty and delivering mail along his route does not present any federal question and administration of the work of the Post Office Department does not require a carrier, while delivering mail, to drive his car from a stopped position into the path of an approaching automobile. When he is charged with doing so, his defense is under state law and is not different from that of any other citizen.
Where, on the other hand, the Federal employee could not discharge his duties without violating State or local traffic regulations, it has been that he is immune from any liability under State or local law for such noncompliance.
Basically, the whole case boils down to this: Federal agents cannot violate state laws and are not immune to prosecution normally. However, when there is no other choice, then the agent is held immune for a particular act. In the case it mentions a Federal agent in pursuit of a suspect that broke traffic laws and ended up hitting someone with his vehicle. He is held immune to that, the same way a cop or fire engine would be held immune for speeding and running lights on their way to a scene.
None of this has anything to do with the discussion of the TSA. Try again.