> 1.5 GB information

Well, nature has a big advantage over us in that it doesn't need to "make sense" of that code :). So it can, for example, do crazy reusage optimization patterns. A "subroutine" that is used in one place could also be part of a "data piece" of another part. A "header" part can also double down as a "validator" of another part. Doesn't need to make sense, it just needs to work. The only limits are the laws of physics. I would not even call it compression at this point. It's more like heavily optimized spaghetti code.

I find using the term "junk DNA" for non-coding DNA to off-putting for exactly this reason. There's most certainly "cruft" accumulated in any evolved organism's DNA, but the very presence of that "cruft" might just as well be serving another purpose.