I think that the right statement he should have used is investing with an amount of money you can do away without for a very long period of time, because Bitcoin investment is not gambling whereby you gamble with an amount you can afford to lose, truly long are those days when investors are investing with fear that Bitcoin will not be worth it, but as of today, even haters of Bitcoin knows that it's steadily on the rise, it's just a matter of time, so it's best to use the statement of investing an amount you can do away with for a very long time, than to say invest with an amount you are willing to lose.
Money that you can afford to lose doesn't mean you're losing it. Bitcoin is in a position where it's virtually impossible to lose it. Money that you can afford to lose is money that you'll never need. But you can use that money after you reach your goal. If you use money that you'll need after a certain period of time, no matter how long that period is, it can force you to make unexpected sales. Suppose you use money that you won't need for 10 years and your investment goal is 10 years. What if you want to extend your investment after 10 years? If you use money that you don't need, you can extend the new term of your investment.
Another reason to invest money that you can afford to lose is that if you accidentally forget the seed part of your wallet, lose access, then your money will be lost, are you ready for that time? It is best to invest money that you can afford to lose, taking all aspects into consideration, so that even if the investment is lost someday, it does not have any negative impact on real life.