Well ain't that a pretty coincidence...
But seriously, there is no one solid theory explaining the appearance of the "lost" items
Random trash wormhole? - but all the items look clean and are clearly usable
Someone purposefully sending items? - But why is there no attempt at communication
They were all brought over by another isekai'd person? - but the items are found all over the continent, in human and demon territories, and no historical evidence beyond the book and even that can be explained away, not to mention all the items are in good condition (despite many years passing)
Divine intervention, to attempt to upgrade the world culturally or technologically? - Could've picked better items than the odd gas canister, granola bar, and plastic bottle
MC unconsciously wishing them into existence, or influencing their random appearance somehow? - seems to be the most cop-out but the most interesting
Don't forget to add more!