Just because you are hiring a lead doesn't mean it's a greenfield project with nothing established; even if the lead has freedom to migrate over time, that doesn't mean that it isn't important for them to be able to guide and mentor on the existing stack very quickly.
I generally agree the I industry overemphasized experience in using particular tools rather than skill in engineering systems, but it is not the case that specific tool (including language) skill is completely irrelevant.
Another exception: contractors. In this case, you do want someone who is immediately productive in the language you're using because you do not reap any of the benefits of them ramping up like you do with permanent employees. For contractors, familiarity with the language and ecosystem is the more important part of their job; for employees, familiarity with the business is more important and the language stuff can be picked up.
The exception to this is if you're hiring the lead for a project, who will be setting the idioms and mentoring the rest of the team. But even then, why are you dictating technology to that person rather than letting them choose?
It is worth asking people whether they are interested / willing to learn the tools you're building on, but it doesn't make sense as a filter.