The investopedia page is rather coy,
> There are conflicting etymologies for this phrase, but the one closest to its current business connotation is the idea of Japanese people loosening their kimonos to relax at home, much like loosening a tie. [1]
Women in Japan haven't exactly enjoyed equal status in the western sense and there is a boat load of subjugation that probably went into coming up with this phrase. The worst example is probably the use of "comfort women" [2] by the Japanese army in WW II. These women were enslaved for sex.
So of course a woman would find this offensive, particularly when directed at her, since if you know any history it is easy to imagine this phrase originating from "open your kimono and have sex with me." I can also imagine oblivious men in tech repeating it if they didn't "get it" at first because they had heard other men use the phrase without realizing its origin. However you do deserve to be called out if you continue using it while feigning ignorance. Your response should simply be, "oh, I'm sorry, I didn't realize the implications of this phrase. I'll stop using it." And if you can't do that then HR should get involved.