Apple's battery fault detection is pretty good actually. Did the battery said "service battery" or "replace battery" while it was operational? If the battery status showed something similar, the battery is already damaged before storage.
I have an 2008 MacBook pro, and one of my older batteries have shown the warning. After a year or so that battery has also swollen, albeit in a milder manner than yours.
I've seen an iPad that acted completely normal despite the case separating due to oxygen expansion while under warranty. Worst part, the owner had to get rid of it since she was on vacation where a Apple store didn't exist.
I have an 2008 MacBook pro, and one of my older batteries have shown the warning. After a year or so that battery has also swollen, albeit in a milder manner than yours.