Why not let them use LLMs? LLM's are a tool for the job so you wind the find the candidate that can most effectively use that tool in your role. If LLM's struggle with your technologies and API's then a developer that can use an LLM for development with good results should be a desirable thing, right?
Can the senior developer understand and internalize your codebase? Can they solve complex problems? If you're paying them to be a senior developer, it likely isn't worth their time to concern themselves with basic data structures when they are trying to solve more complex problems.
You literally asked the question the GP pre-emptively answered. Read their last paragraph.
>Can the senior developer understand and internalize your codebase?
Would you trust someone who needs llms in the hiring phase to be able to do this higher order tasks if they can't nail down the fundamentals?