Bitcoin investment is risky just like any other investment but the risk is lesser and if you invested with a discretionary income, there is no need to think about the risk that is associated to it again since you can bear any aftermath of what comes out from your investment if things peradventure goes wrong
You thought wrongfully about it.
Invest what you can afford to lose is a very classic advice and by invest your discretionary income, you do your capital management better.
Risk is risk and it is nearly the same for your investment capital, and if you picked a wrong cryptocurrency for investment, you will have same 80% loss of your initial investment capital. It's not matter that your initial capital is $100 or $1,000, it gives you same -80% ROI, that is insane and painful.
First of all, I will start by disagree with your statement that risk is risk , no, risk are not risk, their are different level of risk attached to every investment out there, you can't compare the risk attached to Bitcoin investment to the risk attached to alt and meme coin, these are asset with higher level risk, just like gambling, and even the probability of you making something out of it is very slime, so why would you compare such asset to Bitcoin when it comes to risk?
Bitcoin is a volatile asset that's correct, but the risk attached to it can be shrinked to the bearest minimum if it can be held for a very long period of time, so try to know the difference.
Finally, talking about the terms invest what you can afford to lose, they are talking about you investing an amount of money that wouldn't be seen as anything to you, money that if it's lost, you wouldn't be bothered, so that if you investment goes against you, you wouldn't feel much of the impact on your finances.