The lesson is specifically about the use of 'marron' as an adjective for a colour - in this context it is invariable as stated.
There are other meanings for 'marron' as a noun or adjective, and it can be variable in those contexts, although use in the feminine form is rare, according to Larousse.
here is an easy answer for you (I hope!) Marron is a nut. Noisette is also a nut. They are nouns and used as adjectives for color.
(from a French native teacher)
However, marron has meaning beyond just being a nut, or a colour. Hence the dictionary links.
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