Bitcoin is not like any other altcoins or shitcoins. So attributing it with the clause, Invest what you can afford to loose is nearly disrespectful to the reputation of bitcoin. This statement also tend to refer to every bitcoin investor as Bitcoin Traders which is not true. Bitcoin traders gamble bitcoin and just like every other gambling, they can loose anything at any time. Buying to hold is the only way you can be assured of having a safe investment. Bitcoin market could fluctuate any moment and that is when panic sellers loose their money often times.
this phrase invest what you can afford to loose in bitcoin is a financial advise given to investors who are investing in bitcoin, since we don't know what will become of bitcoin in the future it is advisable to invest what you can afford to loose in bitcoin. And secondly that phrase doesn't make bitcoin investors look like traders just as I said earlier it is a financial advise okay.
Somehow that phrase still valid especially if we give those advice to a complete newbie which doesn't have an experience to deal with Bitcoin.
But if we discuss this matter to a experience Bitcoin investor then provably that we could agree and adopt on the word made popular by Michael Saylor which is
I only buy bitcoin with money I can't afford to lose.
If you are a HODLer you could learn a lot from Michael Saylor if you follow him since somehow his belief on Bitcoin is really amazing.
But any of it as long as the intention is to hold for long term and target is to accumulate lots of Bitcoin I think everything is all fine.