TLDR: The more severe the ADHD to more you need to externalise but at the same the harder it is for you to use the tools.

Can confirm, I have severe ADHD and because of that even externalising things to notes/planners/reminders etc. does not for me because I can't keep track of anything or remember to write stuff down, remember to read it etc. And because I have severe ADHD I need to externalise more things than people with mild ADHD so the difficulty to use that stuff increases exponentially as you look at more and more severe ADHD cases.

I hope I got my point across because I can't think right now.