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.
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.