That's where you lost the argument. Women passing as men to serve in armies and do other men's only tasks were very rarely "transitioning". They were at most crossdressing and disguising themselves in order to bypass social restrictions in what they could do. Once they left those male-only spaces, the overwhelming majority of these women identified themselves as women and very often moved on to marry men and have children. They usually wore men's clothes and adopted men's mannerisms and often said they were "as good" as men (which in itself was a scandalous claim in the past), but once the jig was up they weren't trying to convince anyone that they weren't women. Instead, they usually argued that the restrictions placed against women were unwarranted. Consider them not as transgender transitioning, but as proto-feminists challenging gender roles.
Were there transgender women in the past? Most likely, yes. Dysphoria (and body dysmorphic disorder) encompasses many more aspects than just gender, and it is a mental illness that has been with us since forever. But that's not the example you want to use.
EDIT: those cases were "significant" generally because of the stink they raised. But remember that dysphoria, particularly when left untreated by therapy and/or drugs, has a very high rate of comorbid mental conditions (and suicide!). It would have been very difficult for someone with serious enough gender dysphoria to want to transition to have lived a "normal", productive or simply long life in the 19th and early 20th century.
