It's the definition that (as I understand it) most pan and non-queer folk use. I'm pan, and it's the definition that I use. But many in the bi community will claim that bi means (and has always meant) that you're attracted to "your own gender and not your own gender." I recall once seeing a rather old source (from the '90s or possibly earlier) where this was explicitly stated, though I can't remember where I saw that.

But like, if that's all true, then the bi community has done the worst job marketing themselves, even within the queer community. I used to identify as bi. I had no idea. I always thought it was as you described, which is why I started describing myself as pan.

So to specifically answer your question, they typically argue that there is no difference and the term pansexual doesn't need to exist since bisexual already covers it. I don't agree, although I do like the colors of the bi flag much better.