My guess is that the "500 million lines," to the extent it isn't pure fiction, is not actual program logic. Rather, it's data files capturing the legal and regulatory distinctions between the 50 states, all the different providers, etc.
I am aware that writing templates, or using LESS or SCSS can let you generate more lines in output from a much smaller input, but they can't be counting the final output as what will have to be re-written.
I'm not mathematician, but let's say 500 lines of usable code is a good day, that means for this project that would require one million days worth of 'hitting it hard' and not doing any graphics work, correspondence, or any extraneous tasks - JUST code writing.
The timeline for this project was what, 3 years? So in order for 1 million human-days of writing code to take place in 3 years (1095 days), that would require what, 913 monkeys bashing away at keyboards non-stop for 8 hours a day? I mean it is feasible, but that's a gargantuan undertaking and I can't see how something so large could possibly be designed to run an application with so few features.
I worked on a game once that was about 4 million lines, and it had continuously evolved for about a decade at that point.
If the 500 million line comment is true, it probably counts all the lines in all the supporting documentation, xml configuration files (which could be huge), etc.
I'd bet my life's savings that it's not actually half a billion lines of executable code.
500M seems impossible.
EDIT: It occurs to me that I'm only counting code we wrote ourselves. We, unsurprisingly, use a bunch of open source libraries & third party tools. If you counted up all the lines of code in all those things maybe you get to 500M? Maybe?
In any case, the figure seems outrageous. If it is that long, I think the site is doomed, and will have to be rewritten from the top.
Still, I agree the figure is outrageous.
Another way to get there could be metaprogramming. If you have a Java code writing C code writing web pages [1], you'll get progressively larger number of lines of code depending on where you take the count.
Another way could be needless, but legally mandated, duplication. For example since insurance is state-regulated, what if the law mandates data isolation per state--so there are 36 separate data stores: one for each state. That would give you 36 times as many "lines of code" if you were extremely literal in adding it all up.
These are all silly of course. But everything surrounding PPACA seems like it has to be silly, so why not this?
[1] I don't actually believe this is the stack.