That's just assessing someone's ability to design a solution, while presumably what you are hiring for is the proven ability to deliver solutions, add value to the organization over time, etc.

Of course it would be a major red flag if someone couldn't come up with a good sounding solution to something they claimed to be capable of, but a having a plan is not enough. Ideas are really a dime a dozen, and the ability to deliver is the what you really need, which is why VC's tend to fund people, not ideas.