It doesn't seem to be, but it would be difficult to know for certain. If it can be demonstrated that this orphaned other blocks, that might increase the suspicion, but even then it would be difficult to prove with any certainty that it was any type of bock withholding attack.
Would it be fair to say that "technically" it would be possible and block withholding should be classed as attack or exploit should bitcoin not look to implement something like MBBM (must be broadcast message) to coin base transaction to stop block withholding seems very unfair there is even a way to do this in the current system and that there seems to be little or no talk on detection of unfair play in the space.
