Liang got prison time because he _did understand_ that the engine wasn't compliant with regulations and chose to build the system to falsify the emissions output during tests anyway. He was not a scapegoat.
"On 9 September 2016, James Robert Liang, a Volkswagen engineer working at Volkswagen's testing facility in Oxnard, California, admitted as part of a plea deal with the US Department of Justice that the defeat device had been purposely installed in US vehicles with the knowledge of his engineering team: 'Liang admitted that beginning in about 2006, he and his co-conspirators started to design a new "EA 189" diesel engine for sale in the United States. ... When he and his co-conspirators realized that they could not design a diesel engine that would meet the stricter US emissions standards, they designed and implemented [the defeat device] software.'" from https://en.wikipedia.org/wiki/Volkswagen_emissions_scandal