Well, seems fair to ask. It could already have been certified, as far as they know.
The maintainer could respond with an offer to underake certification and bill for his time, or invite them to try get it certified on their own on account of the project being open source.
I do not consider it a reasonable request. The burden to maintain an open-source project is already heavy and entitled users should be flagged whenever possible.
If they pushed or demanded it, it would definitely be another matter, but this was a simple, albeit curt, request, that doesn't even need to be met with a response to be honest, if the maintainer doesn't feel like it. I've seen far worse.
As for your example, I'm sorry, but I'm not impressed by maintainers who act like fainting divas because people don't use the right tone with them. That's just thin skinned.
The maintainer could respond with an offer to underake certification and bill for his time, or invite them to try get it certified on their own on account of the project being open source.