Without looking, I'd guess he has enough service time to decline an option.
However... he signed a minor league deal. He isn't on the 40 man roster, so options are a moot point.
I don't disagree that the bar is low and this is a good situation for him (probably better than his other options). But if you are a GM and have the option to keep 3 players (Tellez, Huira, Voit) or cut one of Tellez/Huira just to put someone in that isn't a clear upgrade... why would you cut one?
Now, when Voit's opt-out clause comes on his minor league deal (I'd bet $$ he has one), now you have a more interesting discussion of which of the two you want to keep. But I really doubt management is going to make the decision prematurely... unless a trade/injury forces it.
The opening day decision in this case isn't so much about "who is better" but what gives us more flexibility.