It's definitely the case that some of the ELF header fields can be repurposed to store additional data, the following articles all address that optimization:

http://www.muppetlabs.com/~breadbox/software/tiny/teensy.htm... https://www.pimzero.com/2020/04/19/golf_so.html https://rpis.ec/blog/plaidctf-golfso/

There are also a few instructions in the article that are size-suboptimal (e.g. "movl $4, %eax" is 5 bytes (b804000000), while the equivalent "xor %eax, %eax; movb $4, %al" is only 4 bytes (31c0b004)).