I generally don't reveal much about my current job on a blog post for security reasons but I can explain some of the reasoning for having your own build system.

We have many packages internally so inevitably we need to build them which necessitates an entire build matrix on it's own. But once you have your own build matrix adding a new package is trivial and most build in under 10 minutes in github actions. One issue is we don't want to use Canonical's default gcc flags so like Gentoo I setup a make.conf that is specific to the suite (22.04, 24.04, etc). This has allowed me to target a higher tier instruction set which is it's own micro optimization. Folks that upgrade blindly from 22.04 to 24.04 will see their performance drop from gcc compile flags alone. Also we have people who have access to CVE fixes before they are generally available so that's also another important reason. Anyway we DO use upstream most of the time for setting up a project folder or normalizing it against a new suite but then building it is using different gcc flags and might be pulling in our version of curl or something.