Darth Ferus said:
Niman, when fully mastered and dedicated to (on the blade side) more or less masters the forms before it.
No.
Niman, when mastered (which takes ten years of constant practice as stated in canon), provides you with a versatile tool that has elements of Shii-Cho, Soresu, Ataru, and Shien. It explicitly does not include Makashi elements, and was created before Djem So was a thing. So at most you could claim to have access to four forms.
Furthermore, Niman's bladework relies heavily on improvisation to make up for the form's weaknesses (which are its extreme complexity at higher levels and the dedication of training needed to master it), meaning that a given Niman duelist's abilities will be sloppier than a dedicated specialist of other forms. So at most, I'd say "an experienced master of Niman more or less has a firm understanding of four of the forms before it."
Niman is not the unbeatable "I know everything" form. It's a form that's easy to pick up on and apply to almost any situation, and it's a form which has an answer to more problems than most other forms. It is
not a form comprised of the entirety of everything short of Vapaad.