By "modular", Apple probably means that it is easier for themselves to provide multiple configurations and make updates, rather than end-users having the ability to stick in random cards/drives/memory/CPUs.
Their other answers saying they want to deliver regular updates speaks to what you said, but the part where he actually mentions modular just reads to me as reassuring people that it's not going to be a higher end SKU of an iMac.
>As part of doing a new Mac Pro — it is, by definition, a modular system — we will be doing a pro display as well. Now you won’t see any of those products this year; we’re in the process of that. We think it’s really important to create something great for our pro customers who want a Mac Pro modular system, and that’ll take longer than this year to do.
>I think, as you talk about the pro user, the fact that our user base is split over notebooks, all-in-one desktops and modular desktops is important. We aren’t making one machine for pros. We’re making three different designs for pros. We’re going to continue to.
>We care about our Pro users who use MacBook Pros, who use iMacs and who use Mac Pros, who use modular systems as well as all-in-one systems, who use the pro software we make.
[1] https://techcrunch.com/2017/04/06/transcript-phil-schiller-c...