Very reasonable that they forbid to sell purely their code, but they allow to use it in any other commercial product and sell those products based on their software.
No, they don't. A quote from their license, with emphasis added:
For purposes of the foregoing, "Sell" means practicing any or all of the rights granted to you under the License to provide to third parties, for a fee or other consideration (including without limitation fees for hosting or consulting/ support services related to the Software), a product or service whose value derives, entirely or substantially, from the functionality of the Software. Any license notice or attribution required by the License must also include this Commons Clause License Condition notice.
You cannot use any part of their code in any other product which you then sell. Thus, it is not open source.
Again, according to the definition of MIT-licence the software which is liable to it is open source. I have never encountered the contradictions to this coming from reputable sources.
How about the source of the people who wrote the Commons Clause license Coldcard use in the first place:
Is this “Open Source”?
No.
“Open source”, has a specific definition that was written years ago and is stewarded by the Open Source Initiative, which approves Open Source licenses. Applying the Commons Clause to an open source project will mean the source code is available, and meets many of the elements of the Open Source Definition, such as free access to source code, freedom to modify, and freedom to re-distribute, but not all of them. So to avoid confusion, it is best not to call Commons Clause software “open source.”