From your link:
Multi-Stream Transport is a feature first introduced in the DisplayPort 1.2 standard.
From this thread:
SoC supports DisplayPort 1.3 (up to 4K@60)
I guess there is a chance that it could work!
BTW, Purism themselves don’t know it seems: Librem 14 USB C Port Dual Monitor Capabilities - #7 by nicole.faerber.