Unfortunately this actually disproves your point because groovy cat says your hero power gives your hero 1 more attack ie your hero receives 1 more attack from your hero power in which case, using your same logic the hero power gives +1 attack, your hero receives +2. The outgoing attack buff shouldn’t change but the incoming buff to your hero should. However the way groovy cat is programmed is that it increases the +attack buff number of your hero power by 1 which based on the wording of Artanis’ hero power should give that buff to both the minion and the hero.
So while I agree with Artanis’ hero power is correctly buffing both targets based on how groovy cat works. Groovy cat is the one that is actually worded incorrectly for how it works, it should read as: Your hero power gives +1 extra attack this game. On a side note this technically also does fix another unrelated issue with the wording because technically as it is currently worded if you are not a druid and summon a groovy cat your hero power should give your hero 1 attack whenever you use it because other hero powers (except demon hunter) give your hero 0 attack and since groovy cat reads to give 1 more attack, other hero powers should technically give your hero 1 attack, but I digress.
So ultimately Artanis is working correctly for how groovy cat is programmed but at the same time groovy cat is not worded correctly for how it is actually programmed.