Do we reckon this is legal?

[Fazerider]

Straight throughs can pass and be legal, no?

Yes, but it depends on how the internals are arranged. A gun silencer does an impressive job of keeping the dBs down and is obviously a straight through design.
That can of yours has a mounting hole for a baffle on the one port fully visible in your pic, I'm willing to bet there's a corresponding one on the other port. If it could pass the noise restrictions without the baffles in... what would be the point of them?