The author of the article claims that a mere migration to a new platform does not solve the problem. It just fragments the community. I agree with that. For one or another reason not all people will migrate.
I can try to think of a simple scenario. For whatever reason, a user may not be as active in the new platform as they were on discord. This could alter the community dynamics. On a bigger scale this could have visible effects in the actual community.
To be clear, I am not advocating in favor of staying on discord. I just find the concept of community building interesting.