Those slides are from 2017. P2P Matrix was released in June 2020. A lot of work is being done on Dendrite, the latest commit was posted two hours ago as of this writing. From the GitHub page for Dendrite: "As of November 2020 we're at around 58% CS API coverage and 83% Federation coverage, though check CI for the latest numbers."

So, yes, for now the metadata leakage is a real issue. However this is likely to change in the near future.