That's not true, I tried last week.
Something like that:
1. Proceed through ordinary steps until account screen.
2. Choose "Set up for work or school".
3. Choose "Sign-in options" and select "Domain Join".
4. Enter username, password, recovery questions.
That's it. Despite the name, the domain join is not happening at installation time. I assume user can join domain once the installation is complete, but it's not enforced or even signalled in any way.