What I think a lot of the comments are missing here is that many companies who use Figma are already using Figma to create and present slide decks. (The company I work for is a perfect example.) It's maybe not the intended use of their software, but it's a very popular one.
So the competitive angle here isn't stealing market share from Google Slides/PPT, or trying to get new users to pick this over other web presentation tools. It's adding this as a first-class use case for people already building slides in Figma to further ingrain the 'ecosystem'.