Which they address in the later part of their answer which you leave out:
Surely most parts of the project could be reused, but this development would still mean a huge investment, which we can't afford. Nevertheless all kinds of contribution are still welcome and we would try to provide our support as good as we can.
It just didn't jibe with me and still feels like it is an easy and obvious upgrade.
But, you are right, they did justify it, it seems like an overstatement to say it would be a huge investment. I should review the code myself to verify, but a statement like that the lazy programmer in me shy away from even doing that.
And i can imagine that maybe they didn't abstract communication with Github enough and would need to refactor the system to handle that as well.
Generally, i think it's not totally reasonable to expect them to do more free work to support use cases that the maintainer does not need. Since it's open source, we're all welcome to contribute back.