Open source software workflows are very common and provide a _lot_ of tooling, e.g., Maven, bundler, npm, etc. Add semantic versioning and you have a lot of tooling that you basically get for free for polyrepo setups. With monorepos, you have to really spend a lot of time tooling, because you basically don't use the OSS tools.
There's a lot of odd arguments in this blog that are very spurious:
"If an organization wishes to create or easily consume OSS, using a polyrepo is required."
What? _Consuming_ OSS is usually not that bad. I've even imported the complete history from external repos, pretty easily. (It does suck with git but I wouldn't use git for a monorepo...) _Contributing_ to OSS is tricky, but the fact you use a polyrepos don't really help you much there either.
"Polyrepo code layout offers clear team/project/abstraction/ownership boundaries and encourages developers to think carefully about contracts."
Clear ownership boundaries has _zero_ to do with polyrepos. In fact, I'd say monorepos can be easier, since you say "everything under this directory is owned by X,Y,Z". There's no search function that's required to figure out where some other team hid their code. So many times, with polyrepos, projects are _hidden_ because they're off in some other grouping unit that you're not a member of. So you don't even know who owns what or where it came from.
In the end, I'd still strongly recommend using polyrepos because you get _a lot_ of tooling for free, and most integration issues are solved with semantic version locking and CD automation. But the arguments here are not really great.