There are several problems with this theory. Firstly, there is no indication that Peter's words are poetic. Given that he says, "do not be ignorant of this one thing", it would seem that he is talking facts, not "spiritual language". The NASB actually uses the word "fact" there.
Secondly, the fact that the statement is presented both ways negates applying just half of it to Adam's situation. In other words, you can't have it both ways.
Thirdly, this is talking about God, not mankind. Fourthly, this is talking about God's patience, not about any part of our life.
Fifthly, this is a metaphor, a comparison which, in this case, uses "as" to relate one concept to another, without saying that one thing is actually another thing.
Sixthly, this kind of reasoning undermines every use of "day" in the Bible... including the ones in Genesis 1 and Exodus 20.