Is a professor who's never had to write a grant application disadvantaged in the job market?

No, this is not a concern.

• Typically individuals who receive endowed professorships already have a long record of applying for grants. You could, as the donor, attempt to negotiate some other situation if you wished.
• Typically endowed professors only leave their job due to retirement or death. If you have really endowed the professorship with enough money, there will be no desire to leave for another endowed professorship, if one could be found. An endowed professorship does not make the holder uncompetitive on the job market. It makes employers uncompetative to hire the professor.
• Most endowed professors do apply for grants.

Assuming you really have that much money to give, my advice would be: Require the holder of the professorship to refrain from applying for grants. This will free up a lot of their time to do something more productive.

If you want to make a positive impact on academia, you can achieve more for your dollar by endowing (or just spending all the money immediately on) scholarships at institutions that charge low tuition, like community colleges.

Let me focus on the headline question: Is a professor who's never had to write a grant application disadvantaged in the job market?. I'll leave aside your admirable intention to provide funding.

To do their job, a professor needs many skills, but acquiring them takes time and effort. Not all of us are (in my case, were) excellent teachers or even researchers at the start of our careers and had to learn those skills and others over time. Obtaining grants is one of those valued and valuable skills.

But note that such grants aren't solely to provide salary and benefits to the grant recipient. Typically they fund many more things, the most important of which is probably support for students. But they also cover such things as travel, conference and publication fees, lab equipment (and maintenance). In some fields there are also technicians that need to be employed. Travel and the opportunity that provides for collaborative work can be very important. Grants are also highly valued by universities, since the "overhead" charged against the grant funds such things as lab space (including maintenance) and support staff, including the necessary legal and administrative costs.

So, yes, if a person never acquires this skill they will be disadvantaged in academia generally, though as Anonymous Physicist suggests, a person would be unlikely to leave a generously funded position. A "beginning of career" person can't, of course, be expected to have such skills, but, at almost every institution, would be expected to work to gain them. And failure of a grant application is just a learning experience, like falling off your bike several times while trying to learn to ride it as a kid. You often will get feedback on such failures that help you on the next one.