I just tested it. Replay IDs #15507408 and #15507454
The Cybran Siren-class cruiser can assist the construction of buildings. I tested it with underwater mex upgrades and also with the construction of a T3 pgen on land (obviously, right at the water's edge).
It has 1 BP, exactly 1/5 of what a t1 engineer has. For a mex upgrading to t2, it spends 1 mass/second and 6 e/second, which is exactly 20% of what a T1 engie spends (5 mass and 30 e). Similarly, to assist a mex upgrading to t3, the engie uses 8 mass/second, and the siren uses 1.6. so, exactly 20%
The assistance does, as expected, increase the rate at which the upgrades complete
I think this is a bug, there's no in-game or lore reason why Sirens should have this ability.
I wondered if this might be because Sirens have air staging. So I tested other units/structures with air staging to see if they have the same bug (all t3 aircraft carriers, Atlantis, Czar, Fatboy, and the air staging structures). I was not able to replicate this bug with those other units. So there is something else going on with the Siren.
I wonder if the existence of a guard radius ("GuardReturnRadius = 130") makes a difference. Or perhaps it is the targeting laser weapon. Just a couple wild guesses. If I wasn't so lazy I could mod the game to remove the guard radius to test if that's what's causing it.