They are the same thing. No one is saying Reddit bases their entire infrastructure on Nim. The point is just that the language is being successfully used in a business context.
This might not be important to you if you're not a user of the language, but for people that are, it is significant for two reasons: 1) it shows it's 'good enough' to do analytics & data processing for a large, data heavy company and 2) creates confidence/proof it can be used in a business context.
As for technical debt, I doubt that's the case. The post mentions internal tools and processing, these kind of things are often not subject to high developer churn.
Besides, Nim has a pretty easy to read syntax that's similar to Python. People who have talked about onboarding developers to the language (such as https://youtu.be/5wljNaPkU7M?t=586) say it has been fairly straightforward to train for if desired.