There are waaay too many hits on Google on this topic :-) Whoever wrote this sentence may have heard the story posted by the Nürnberger Zeitung in 2010 called "Oh You Holy Potato" referring to two seperate families who both cut into a potato to find the sign of the cross. What?!
This is one explanation from Google:
Urban Dictionary: Holy Potato https://www.urbandictionary.com/define.php?term=Holy Potato Holy Potatounknown. The Holy Potato is made up of three minor potatoes. The Baked Potato, The Sweet Potato, and The Unchanged Potato. The Baked Potato is the being of willpower, The Sweet Potato is the master of wisdom, and The Unchanged Potato is the mind of knowledge. These three spirits combined make the Holy Potato, one organism that lives is the dimension between dimensions.
Ich finde es komsich! "The holy potato"; "They wanted to see the potato"! Duo carries on with this playful innuendo as if they're hinting at something else rather than a garden vegetable i.e. as if it's a code name for some illegal stuff! This cracks me up every time there's a mention of "a potato"! A holy potato, of all things!