Coldcard also built on many open source libraries (not just Trezor's) when they designed their product. For them to start whining about people building up their open source library is just pure hypocrisy.
Are they even allowed to change from GPL to MIT license?
GNU General Public License (GPL): The GPL is one of the most well-known open source licenses. It is considered a restrictive license, as it requires that any changes made to the code must be released under the same GPL license, and any software that uses the code must also be released under the same GPL license. Additionally, if a user distributes the software, they must also provide the source code and any changes they made to it.
So if they're building on other GPL software, they have to keep the same license for their own software.