The motherboard maker send out a press release today to clarify the matter. MSI says they're still performing tests on their current 300- and 400-series motherboards, but that they intend to offer maximum compatibility:
It has come to our attention that MSI Customer Support has regrettably misinformed an MSI customer with regards to potential support for next-gen AMD CPUs on the MSI X370 XPOWER GAMING TITANIUM motherboard. Through this statement we want clarify the current situation.
At this point, we are still performing extensive testing on our existing lineup of 300- and 400-series AM4 motherboards to verify potential compatibility for the next-gen AMD Ryzen CPUs. To be clear: Our intention is to offer maximum compatibility for as many MSI products as possible. Towards the launch of the next-gen AMD CPUs, we will release a compatibility list of MSI AM4 motherboards. Below is a full list of upcoming BIOS versions which include compatibility for the next-gen AMD APUs for our 300-Series and 400-Series AM4 motherboards based on the latest AMD Combo PI version 188.8.131.52. These BIOS versions are expected to be released in May this year.