'Mental fitness' does not destimatize 'mental health', it's the same thing. The subject that really needs destigmatizing is mental illness.
One of the problems with untrained amateurs taking about mental anything is that they treat everything the same and think everything can be treated the same. Services like Better Up, Counsellors, online support etc are fine if you are going through a dip in your mental health/ fitness, if you are under stress or feeling sad. But if you are in the throws of a mental illness: clinical depression, diagnosed anxiety disorder, bipolar, schizophrenia, etc they are useless and best and harmful for many.
Serious mental illness needs medically trained psychiatrists, clinical psychologists, doctors, not an online check and instructions to go for a walk or learn how to breathe.
Those things are not bad and they are helpful for maintaining mental health but they do nothing for when you are seeing demons in the trees. When you are under a compulsion to harm yourself. I could give other examples but they would be too outing.
I'm sure Harry means well but he has no medical training and he needs to be clear about what he can offer; suggestions for supporting your mental wellbeing and what he is not qualified to talk about; managing mental illness.
This subject is really close to home for me and I get on a bit of a soap box. It's also too complex for one online comment. But the problems I'm drawing with a very broad brush are not limited to Harry but in the wider society and it's approach to mental health and mental illness. There has been progress in talking about some things but others are being either ignored or trivialised.