I think you're misunderstanding things a bit. In the context of DNA as a data storage medium, it no longer functions as a "building block of life". It'd be like taking some binary image data and overwriting it with binary audio data. You can still technically interpret it as image data, but it's just going to be a random jumbled mess, and will probably error out immediately.
But you can also embed interesting information into an image without distorting the image itself (much). Similarly, the DNA of most organisms have a ton of garbage space that could be used to encode information without hurting the organism.
Not saying it's practical or desirable, just that it is possible.
Leads me to wonder if the current "garbage" sequences in our own DNA could be encoded information that we were meant to decode - and by that I mean that that information may not just be entirely genetic code.
They are not garbage sequences iirc, they control gene expression. DNA is a string that is coiled tight several times over itself (can't explain better than that, see the video below :)). So the garbage sequence is like padding that pushes genes in and out of areas that can be read. Changing amount or composition(?) of the garbage can turn a gene on or off just because it becomes inaccessible to RNA replication mechanism.
