In an ideal scenario, you don't use buckets "customers", "competitors" etc. You define them in a sentence, e.g., if you are entering a market: "Next, I'd like to find out if there is a growing customer niche which we can serve". When we say "customers", case interviewers understand this is a common practice used to remain efficient in a limited time frame. However, when working with clients and on full projects, you don't just use words "customers, "competitors" for buckets. That would be a vague and poor communication.
With this in mind, there is nothing wrong with using the bucket "market" separately from what is normally considered market sub-buckets (customers, competitors). When explaining your structure, you, of course, will elaborate what your market bucket includes and those bullets shouldn't overlap with your other bullets. You might discuss general market change patterns in markets and go deeper into customer preferences in customers. You might want to discuss small bullets that shouldn't be separate buckets but are still worth mentioning (like in Vlad's post).
In general, you might want to add "market" bucket in market focused cases, e.g., client has a market decline problem, what can we do. There, you can say in market, I'd like to understand how major market dimensions, including customers, products, channels have increased or shrank over time; how has our position changed in each of these areas. This would be an effective helicopter view. Otherwise, you'll waste yourself over 3-5 different large buckets asking like a parrot the same question in each of the buckets. Once you identify, for example, that the problem is in changing customer demographics, you can jump straight into your more detailed customer bucket and eliminate competitors etc.