Usually I'm not that decisive but in this case I totally disagree with the conclusions!

Having shared libraries including custom libraries - is a good thing and the fact that your company's microservices are polyglot is not a problem (it has pros and cons like anything else, but it's not a "problem").

I suspect that the problem in your company is that you don't have good development/staging environments or good E2E tests or good unit-test suites in your custom libraries (or any combination of the above).

If you had all three, issues in custom-libraries would be easily get surfaced and caught before reaching production and wouldn't be an issue.