The backdoor was not contained within the source code, but within precompiled binary blobs sent "downstream" from the maintainer, this is often done so that end users get a leaner version of the software without development tool chains attached, which also makes automated checking of these blobs difficult to impossible so instead we rely on verified and trusted upstream maintainers to be "good actors". That's the reason this is such a big wakeup call, as it's a maintainer that worked on projects and waited for years before trying to push this through.