Immersion cooled sealed electronics has existed for many decades, for instance in military or aviation applications. They are problematic and costly to produce, because pressure can build up in a sealed system.Sealed systems usually also focus on smaller pieces of hardware with a lower power density, leading to the duplication of parts in use.
Example: 20 main boards would require 20 sealed enclosures, 20 heat exchangers, 40 water connectors (in and out), 20 power power connectors, 20 monitoring devices, etc. All these parts would be very difficult to service. In an open bath immersion cooling approach, with 2-phase cooling, the enclosure is semi-open, doesn't have to be sealed, and said parts are all avoided because only one enclosure exists. Easier to service, less expensive and complex to build.
Similar to oil immersion cooling, "sealed servers" are employing a single phase cooling process. This is much less efficient compared to an 2-phase systems (the heat transfer coefficient is much higher) and requires more components. High density such as in passive 2-phase systems would simply not be possible.
From a semantic point of view, sealed servers don't represent a cooling process on its own that can be compared with passive 2-phase immersion cooling. It's more a product line comparable to blade servers.
It's an approach in the right direction to save energy, but it's more an infrastructure approach - in an optimal case, a customer buys a number of servers from a company and sticks them into cabinets it bought form the same company. Compared to blades, one of the downsides is that memory or CPUs can't be replaced in the field easily. Servers need to go back to the vendor or the vendor needs to send somebody to service the hardware. Not so in our open approach.
Technically, sealed servers use a dielectric fluid such as mineral oil or a fluid similar to the liquids used in passive 2-phase immersion cooling. But the heat removal process does not happen through evaporation and condensation.
Sealed servers, compared to air cooling, replace one evil with another (ie. fans and air filters with hydraulic pumps and oil filters or unserviceable parts), while passive 2-phase immersion cooling simply eliminates all these evils. Technically, passive 2-phase immersion cooling is way ahead, and practically we believe the same is also true.