If a validator do that the rewards are indeed sent to that smart contract but the process of validator sweeping does not interact with the contract's fallback function. The rewards are just directly sent to the contract address and does not trigger any code execution, the reason for that is at the protocol level outside of the EVM and it is gas free.
EIP-4895 has a couple explanations to @Saxtheowl's good answer.
Why only balance updates? No general EVM execution?
More general processing introduces the risk of failures, which
complicates accounting on the beacon chain.
This EIP suggests a route for withdrawals that provides most of the
benefits for a minimum of the (complexity) cost.
Why no (gas) costs for the withdrawal type?
The maximum number of withdrawals that can reach the execution layer
at a given time is bounded (enforced by the consensus layer) and this
limit has been chosen so that any execution layer operational costs
are negligible in the context of the broader payload execution.
This bound applies to both computational cost (only a few balance
updates in the state) and storage/networking cost as the additional
payload footprint is kept small (current parameterizations put the
additional overhead at ~1% of current average payload size).