If you don't have a couple of hundreds engineers who would write a custom file system for git (Microsoft), or who would take an existing source control system and nearly fully rewrite it (Facebook), or who would write a custom source control system from scratch (Google, Yandex, etc) DON'T USE MONOREPO.
Otherwise you are risking to end up in a situation when hundreds of your engineers have to spend tens of minutes every time they need to simply push their code changes.
Google did not need to write their own SCM to reach the scale mentioned in the article. Off-the-shelf Perforce served that company well enough when it had tens of thousands of engineers and millions of files. They had 11 million files in a monorepo with one 256GB machine which most of you would consider a bare bones desktop computer at this point.
Of course, you shouldn't start a project from 10 repos. But the downside is while you live in a single repo all the tooling get tuned to work with a single repo. And then there is no way to go multi-repo when you need to.
There is no point in making any argument with someone who has the conviction of their religion behind them. Followers of the monorepo religion are no different.
Otherwise you are risking to end up in a situation when hundreds of your engineers have to spend tens of minutes every time they need to simply push their code changes.