No... no, they're not.
1. Sexuality - particularly with women, in my experience - is variable: hetro women can and do consider other women as viable sexual partners, even if they don't consider them long term partners;
2. The implication of a "surplus of hetero females" is flawed given that bi and homo females do, in fact, exist and aren't properly accounted for. Not every single female in that surplus is available to you. In fact, some of them are available exclusively to "themselves" if you were.
also, not every single female in the surplus needs to be 'available', since it's a statistical implication. a surplus of hetero females correlates with a surplus of 'available' hetero females.
I don't think she's in any way suggesting you're wrong, (and neither am I) she just wants you to remember that she exists as well and benefits from this too. I can't speak for her but I also see it as a slightly humorous addendum to your point.
Could have just stated its good for lesbians too (and included like me), and been done with it, and come off as less hostile.