i think it is perfectly acceptable to refer to a chain tug as a chain tensioner, but is more ambiguous as chain tug is specifically one thing, whereas chain tensioner can refer to both things.
whilst this confusion may be seen by you as error, it probably just arises from abbreviation of "chain tensioner for vertical dropouts" (what you take it to mean) and "chain tensioner for horizontal dropouts" (i.e. chain tugs)
i think it is perfectly acceptable to refer to a chain tug as a chain tensioner, but is more ambiguous as chain tug is specifically one thing, whereas chain tensioner can refer to both things.
even sheldon's shop calls chain tugs chain tensioners.
and so to many others 1 2 3 4 5 etc
whilst this confusion may be seen by you as error, it probably just arises from abbreviation of "chain tensioner for vertical dropouts" (what you take it to mean) and "chain tensioner for horizontal dropouts" (i.e. chain tugs)