Your assumption that a company’s priority is to not share is a possibility, but it is a worst case one, and one that is IMO less of a concern for a programming language: keeping bug fixes or even new language features to themselves may very well be a net negative. A buggy reference language implementation for external users will turn them off from using it, and make it harder to hire experienced users or benefit from improvements by others.

For such a case, the fear of using AGPL is much more likely one of accidental contamination, such as an employee copying a piece of AGPL licensed code into a completely different, unrelated project.