The point is that transsexuals fall into two groups separated by sexual orientation.
What an incredible discovery Blanchard made, that some people fancy women and some people fancy men. He's wrong of course, bisexuality also clearly does exist and there's no room for that in autogynephilia. Autogynephilia demands jumping through so many intellectual hoops when confronted with the real world whereas the idea that people who have some form of gender discordance might end up with a couple of kinks seems very simple and plausible. And it happens to match what most trans people report, so in the absence of over-whelming evidence for Blanchard's theory, and given the large amount of evidence disputing it, then Serano's ideas seem much closer to what is likely to be the truth.