The difference is clear, investing what you can afford is used when you are interested in building future wealth while investing what you can afford to lose applies to cases where you just want a short-term profit and it mostly happens when you're trying things out.
I totally agree with you that investing what you can afford to loose in a business that you have the believe that it will yield profit in the future is not a good business plan. Except when you don't have the believe that the business will last and you are gambling your funds, then it will become a good business plan to invest the amount that you can afford to loose. I don't think that it is proper to say that we should invest the amount that we can afford to loose in a cryptocurrency like Bitcoin, it is a store of value and we can invest the amount that we have budgeted for investment like the DCA method, not necessarily the amount that we can afford to throw away. I can invest the amount that I can afford to loose in altcoins, especially the ones that have not gained reputation in the cryptocurrency market.