> The term was coined by psychiatrist Tamaki Saito in the late 1990s

I interpret the term (n. hikikomori) differently to the verb (hikikomoru) from which it is derived.

The term (hikikomori): individual instance of the social phenomenon of withdrawing from society and living in persistent isolation.

The verb (hikikomoru): used in reference to a specific (non-persistent) act of staying indoors, without necessarily carrying the broader sociological meaning. Edit: as another comment points out, just staying indoors for a weekend, etc.