If you had access to the source code for the full tool chain, viewing that source would indeed be sufficient. The thing is, that is a nigh impossible task decades after the fact. Note that decades after the fact refers not to today, but when the article was written. Assemblers were already a thing, and an assembler is nothing more than a simple compiler.
If you want to be sure that your compiler hasn’t been hacked, you need to write it in binary. And you need to do that for your entire tool chain — perhaps it wasn’t the compiler that was hacked, but the disk driver or editor. Perhaps it was the file copier. At some point you either have to trust your tools, or start from scratch with bare metal. Today’s CPU’s are sophisticated enough that you could hide such a hack in them without difficulty.
Think of the recent Intel chip vulnerability (meltdown, spectre) or the alleged Bloomberg China chip attack.