In short - Archon gains +2+2 only while in your Hand or Deck, not while been on battlefield, which is strange cause Champion DOES gain +1+1 when on the battlefield.
Indeed, a few cards would benefit from a better description. Faeries, for instance, get their buffs only while on the field; while it’s pretty obvious when yo uthink of it, it’s written nowhere. Some cards do have this written clearly, but not all.
Currently, through experience, it’s more or less manageable (fail it once, and then you know) , but relying on memory is ot a good solution, and when more cards will be edited, that won’t be possible anymore.
That problem was already raised by someone in the community which had proposed to create keywords (just like Haste or Taunt) to know when and where they are applicable (for instance : Hand, Field, Deck, Anywhere), and I believe it might be a good solution.