The question changes to: is what you are testing the actual skill you need? Are you hiring a teacher who is going to explain var,let,const as part of their job or are you hiring someone who needs to know how properly to use var,let,const?
Send me over any developer who fails your teaching requirements for var,let,const but has code samples with proper usage.
I've never seen a senior developer fired for poor coding. I've seen them get fired for being part of a department cut or because they got into a fight with the owner/boss. I've seen them fired because they make too much compared to outsourced resources.