This is true. But investment is by choice and willingness. Someone might be financially ready but not mentally he is not. It might be that he needs to know one or two things about Bitcoin investment before jumping into it. However, getting little knowledge does not take long one or two days is enough so that shouldn't be an excuse. I also notice that many persons due to other failed investments they have done in the past still have this fear in them on investing into another asset like Bitcoin. Well, such persons should guided and mentored so that they can overcome the fear and start their investment.
Mental preparation for Bitcoin investment is just an excuse. If you are financially ready for Bitcoin investment and have basic knowledge, then you can invest without hesitation. If you are planning to invest in Bitcoin, it means that you are planning to invest after knowing Bitcoin. So what kind of mental preparation do you need? You should clarify your position about investment at the beginning. If you are negative about Bitcoin, then I would advise you to listen to an experienced Bitcoiner. If you are positive about investment and have basic knowledge like buying Bitcoin, then you should feel free to start investing.
Only three things are needed to start investing 1. Prudence/alternative income source, 2. Basic knowledge and 3. Long-term planning. If you can ensure these three things, then I think you are absolutely ready for investment. You can start investing without any delay. To sustain your investment in the long term, you need to gain knowledge later, which is definitely after you start investing, and you should not delay investing for this knowledge. If you delay investing to learn more beyond basic knowledge, you are making a mistake and missing out on buying opportunities.