They legally own it in the sense that they are the legally registered owners, currently.
They do not legally own it in the sense that the original owner will not be able to eventually recover it after the fraud is unravelled.
Your instinct that this is odd is correct. It is odd because it isn't actually true. The statement of a police officer made while standing on the street outside someone's house deciding whose story to believe is not the final legal verdict on this case.
It is not different in the UK. This person will get their house back when the Land Registry has ticked all the boxes.
They were shocked by the reluctance of the police to enforce their property rights simply on their say-so. They were shocked by it because they hadn't really thought through how the system works. It is not the role of the police to evict someone who is the registered owner merely on the say-so of someone else who claims to have been defrauded. Clearly there are good reasons for this.
The article presents the facts as though they will never get their house back (with some creative ambiguity about what 'legal owner' means - does it mean the legally registered owner, regardless of any past fraud? or the actual rightful owner) because it makes a more interesting article than 'Man left annoyed after a painstaking legal process restores him his property rights'.
If A says that B is trespassing on their property, and A not B is listed as the legal owner of the property according to the single source of truth, isn't it normal that the police should evict B and defend A's property rights?
Imagine if the police took the attitude "we have to give equal weight to B's hard luck story about how he's the technical owner". Harassment and vexatious claims of fraud would be absolutely rampant.
You are confusing who actually owned it, or who morally owned it, with who legally owned it.
The claim is not that, once the legal issues and the fraud get untangled, the buyer will be held to be the rightful owner. The claim is that AT THE MOMENT, while the 'new owner' is listed in the Land Registry as owning it, and the 'old owner' isn't, the 'new owner' temporarily legally owns it.
They have written this article as though to suggest that this is final and the original owner has no recourse. That isn't the case. What is the case is that the police don't have a remit to investigate the fraudulent sale. If person A is listed in the registry (they 'legally own' the property) and person B isn't, the police will follow person A's instructions to remove person B from the property, but not vice versa.
"So you want to do X" is a standard phrase which is used to introduce advice on how to do something, or why not to do it. See the poem "so you want to be a writer" by Charles Bukowski.