That’s exactly the point of end-to-end (or client-side) encryption. They don’t have the key then. This is not to be confused with encryption at rest (most cloud providers do that) and encryption in transit (every provider does that).
Small correction: they are “source available” but not “free”. The license does not allow forking them. Kind of understandable, because they don’t want a competitor to just copy their code and offer the same for less.