The author does say "straight people". If that was some sort of "yuri is just a phase" thing, they wouldn't bother to use the word "straight". They are just trying to justifiy why they still love the pairing, probably, even though they're straight. You have to agree that straight girls having lesbian sex does sound like a mistake of their past. (I don't think straight girls would enjoy lesbian sex, but that's another thing.)

If two girls had lesbian sex in the past, they're probably not straight. Just because a girl falls in love with a guy doesn't mean she's straight. Bisexual/Pansexual people exist.

Further, even supposing two 100% heterosexual girls for whatever reason have sex, it's wrong to call it a mistake unless there are negative consequences. Honestly, it's more likely that it wasn't a mistake, because it's likely that it at least gave them a better understanding of their own sexuality, which can only be a good thing.