I don’t completely agree with that.
While there are a number of excellent writer-directors working in the industry today, there is always something to be said for collaboration.
Rather than trying to do everything yourself, particularly if this is your first project, I would recommend finding a partner to work with who possesses the skills or talent you are lacking.
If you are an accomplished director interested in writing their own material, I’d suggest searching out a writer you respect who is currently working on roughly the same level as you. As you work on the project together, you will both learn from the other.
If you are a writer who’s trying to figure out how the character would respond to a situation, find an actor and have them improvise the scene with you as you write it.
The entertainment business is an ultra-competitive industry, but sometimes, to borrow a quote from the movie A Beautiful Mind:
“If none of us goes for the blonde… It’s the only way to win.”