There might be code later that says that if the anthropic api key is not set, then turn off the LLM feature. Wouldn't make sense for this LLM-related code but the concept makes sense for using various APIs from dev.
But if you do ENV[xxx] the value is also set to nil.
Using .fetch with a default of nil is what's arguably not very useful.
IMO it's just a robocop rule to use .fetch, which is useful in general for exploding on missing configuration but not useful if a missing value is handled.