Even if you could reduce the brain to some sort of bytecode, an interpreter is still necessary to run that bytecode. For instance, a python program might be a few bytes, but the interpreter is still a few megabytes. Yet both are necessary to run the program. Who knows how large a brain bytecode interpreter is going to be, but probably very large.
There are two problems with this argument:
1. For the interpreter to substantially reduce the size of the DNA needed to encode a program to build an intelligent system, that interpreter needs to be optimized to reduce the complexity of such programs. However, as far as we can tell, ribosomes and protein folding work exactly the same way in nearly all living cells, from snottites to kelp. It isn't plausible to suggest that each Salmonella cell contains hundreds of megabytes of information that's optimized for producing intelligent systems. Even if Salmonella contains hundreds of megabytes of information, it would amount to a proof of creationism if that information were optimized to simplify the expression of brain designs. So the size of the interpreter is irrelevant.
2. DNA is not just interpreted; it's compiled into cells. The DNA of a cell contains a complete program for making every peptide that is necessary to the cell from individual amino acids, and those peptides together construct all the other chemicals from a small number of simple molecules found in the environment. So the source code for the "interpreter", down to the hardware level, is actually already present in the DNA.
If one knows python and is given a program in APL, then likely an insurmountable barrier has been reached. If you don't have the docs the describe the language, the one can try to infer the language by running experiments on variations of the stored program. However one needs access to the processor in order to run different experiments and get different results to be able understand how the programming language works.
We don't have CPU in a form that we can experiment with ("brains in a vat"). We have a 50Mb string in APL*2, a mostly unknown language for a mostly unknown processor.
The other part is that this is not a program but a meta-program -- meaning there are multiple levels of indirection. The DNA does not directly specify the brain, but instead specifies rules for components that would eventually arrive at an assembly (guided by a rich context of voluminous other inputs over an extended period of time) that constitutes a brain.