however you are correct because as an investor or a business man is never an ideal to have only one single investment because if anything should happen to it, you will be hit back starting from square one.
To start up any investment is not easy, the only investment that one can get started easily is bitcoin investment because you can start with any amount of money. You should also know that we are talking about beginners, who just want to start their bitcoin journey. Especially low income earners who don't have much to invest due to their little cash inflow, such persons should focus on only buying bitcoin and accumulating with the little amount that he can afford through regular DCA as is the superior to the other two methods, so that he can stay focused on building his bitcoin portfolio towards his bitcoin target. The low income earner wouldn't be able to have other investment and bitcoin investment is still that can promise a high profit in the long run compared to any other investment so why not just focus on the the important one than distracting yourself.
It is when you have reached close to your bitcoin target, that if you want to invest in other investment like bond, stock, gold or real estate that you can now start that, not at the early stage of your bitcoin journey, if not you might end up not achieving your bitcoin goal.
I hope that when you mean other investment, you are not talking about buying shitcoins, along side with bitcoin. If that is what is in your head, they you are misleading yourself to the wrong way of investing because buying shitcoins are waste of time and resources. No need of that because only bitcoin is what you need and forget about any other cryptocurrency because they are the duplicate of bitcoin and they will end up disappointing you and you will regret your actions.