A number of sources believe it's referred to as a "powder room" because it was indelicate for women, in mixed company, to say they were going to the bathroom,

What sources?

'Bathrooms' or 'water closets' didn't really become a prolific thing untill near the end of the 19th century it wasn't untill the first few decades of the 20th century that piped water and sewers to take sewage from your house were expected as part of a basic standard of living rather than luxuries.

Powder rooms have been around in palaces or residential buildings for a long time. When water distribution and sewers started becoming common place around the developed world powder rooms were optimal places to retrofit into a water closet(add in a toilet) or bathrooms(add in tub). Before bathrooms became a thing, if you needed to pee or blast a deuce while a guest at someone's house or palace you'd either go out back to their outhouse, or to their bedroom or some other private place and use a bedpan, if you were at a really fancy place they'd have a hole in their cellar leading to a septic pit.