They actually are quite similar words, for companies which it seems are engaged in the exact same business.

I can easily imagine how someone, not familiar with either company, might hear of Moz and, later, mistakenly believe that the company they heard about was Doz (or vice versa).

Don't forget, trademark law is such that you have to protect your trademark or you lose it. If there's any risk at all of infringement, you have to go after it. So although the case that Moz and Doz are similar isn't particularly strong, it seems close enough that I'm not surprised Moz is taking this action.