"Ownership" of a word is not the issue here. The standard for trademark infringement is based on a "confusingly similar" basis. Unfortunately there is no hard and fast rule here: it's up to a judge to decide. That final points made in the article are exactly how one would go about mounting a defense; but again, you are at the mercy of the courts here and fighting this would cost.

(IMO the PinPigeon logo is indeed confusingly similar)