I'm half-joking here, but one reason we might know it isn't true is the "argument from cash flow": Snap is still hemorrhaging money, to the consternation of investors, mainly because they're paying astronomical sums to Google for GCE hosting. Given the amount of data that passes through Snapchat every day and the cost to host it, I can make a decent guess that they're not hanging on to it, or else their infrastructure costs would be even greater.
Their costs is already in the hundreds of million of dollars. They could hang on to all videos with no issue whatsoever. That's financially easily doable within their budget, and that's technically easily doable because google storage will take any amount of TB thrown at it.
The only fair assumption is that they store all videos. Remember that they are an ad business.
If it's too annoying, the wise decision would be to cut storage to only 1 month, or store only 1% of randomly selected videos. It will never be to stop storing videos.
