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.