It’s not wrong though. They are offering their terms, to which you are providing a counter offer that includes your terms. Standard negotiation practice.
Not following through with the fully executed agreement is what would be dishonest.
The problem I see is that the term is a pretty exceptional one. The employer reasonably thinks you both have been talking about a full-time position.
It's like going through the whole interview process with a company that's clearly hiring in-office positions, and then only after getting an offer, telling them you will only work remotely. It would be different if it were really part of the negotiation - ie they're offering way less than your asking/current salary, and you're countering with part-time for that same salary (or other benefits) to compensate.
For retail transactions we have bait-and-switch laws which address the same type of issue.
Personally I'd be pissed at whoever wasted my time like that, and wouldn't hire them based on the ethics they're displaying. If they brought it up during the interview process - even as just something they're looking for - I'd respect the fact that they gave me the option of considering it as part of the evaluation, giving me the option to clarify and end the process early if it just wouldn't work for the employer.
This is something I couldn't put into words but that was part of my thinking process. I personally would feel really bad if I knowingly hid this crucial bit of information during the process and then made it my requirement.
Maybe in some other work culture you have to be sneaky if you want a part-time job. Here in Finland I'd expect at least one third of IT companies to happily allow 80% time if you just bring it up.
Not following through with the fully executed agreement is what would be dishonest.