Well, they don't have any currently well-selling model whose sales would get ruined by the Osborne effect, and the pre-announcement may keep a few customers from defecting to Windows, Linux, or Hackintoshes.

Schiller told Gruber "it is, by definition, a modular system":

https://daringfireball.net/2017/04/the_mac_pro_lives

I remember they mentioning it during the iMac Pro announcement. I think it was Phil Schiller.