What kind of protection do these Ethernet "surge protection" diodes provide?
If the PCB itself can be trusted, they are diodes.  They also look like diodes.

Wired as they are, they will short any data line to any other if the potential between them reaches twice the breakdown of the diode - the way they are wired, they are effectively in series.

If the cables are grounded, then the diodes will short the over voltage to ground through the shield.

It will probably do something.  Whether it is enough is hard to say.

If you really want protection, use something that you have to connect to an earth ground.  That will dump over voltage to ground rather than through the ground wire on your equipment.