What about the file in the article makes it a "non-ELF binary"? The only thing I can think of is putting junk data in place of bytes the ELF spec designates as

What about the file in the article makes it a "non-ELF binary"? The only thing I can think of is putting junk data in place of bytes the ELF spec designates as "padding" and expects to be 0. Other than that, it seems totally reasonable that putting garbage in place of a section-header offset with no headers, a physical address, and an alignment field wouldn't make it an acceptable ELF. It's entirely on the Linux kernel to not verify these fields. However, its failure to verify these doesn't make the file not an ELF. It just makes it an ELF with a stupid alignment requirement that Linux happens to ignore.