Question

why night reaction is said to be a misnomer

Answer 1

misnomer is a name or term that suggests an idea that is known to be wrong.[1]Misnomers often arise because something was named long before its correct nature was known, or because the nature of an earlier form is no longer the norm. A misnomer may also be simply a word that someone uses incorrectly or misleadingly.[2] The word "misnomer" does not mean "misunderstanding" or "popular misconception",[2] and a number of misnomers remain in common usage — which is to say, the fact of a word being a misnomer does not necessarily make usage of the word incorrect.

Answer 2

The night reaction is a misnomer becasue it doesn't depend on the night time to occur.

Although it uses products generated in the light phase (eg. NADPH,ATP etc.).

So somehow it is indirectly dependent on the light phase.That's why the name night reaction is completely misleading and misnomer.