Choosing shard key with compound index field
You should be able to use the index listed to cover the shard key. It is a super set of your shard key fields. 

The shard key listed should be fine for distributing write load, given you don't expect any individual articleId/host pairs to take the bulk of your writes at a given point in time.

I would be concerned about this shard key for reads. In order to target a single shard for a query, you need to include the shard key values. My guess is your queries do not include timestamp. Without timestamp your queries will be sent to every shard which is inefficient. With scatter gather reads, your hamper your ability to scale reads by adding shards.