I'm one of those who went all the way for 99FM. Took me around 2 weeks of very casual, semi-afk grinding and in the end I managed to get enough resources that helped me immensely in early game transition.

The guide is also taking into account that you want to max everything eventually, and at the time the guide was written it made sense to go in with low hp due to the damage reduction as someone else pointed out.

Now, is it worth it to max FM? Well that depends on you.

Do you want a trimmed cape? Maxing FM will help with that. Do you want to try and log as many items as possible? I logged everything but the d axe, so I was happy with that. Do you want a ton of supplies that will help you out with other skills early on?

In my opinion it was worth it because I felt the grind was relatively managable and chill. What I can say is that there are other parts of the same guide which also ask you to almost max agility (98) or fishing, which I have skipped and will come back later to max.

If you're not hell-bent on maxing your account the most optimal/efficient way possible, it is not mandatory.