Knock unit into water vs transport

Can any one clarify if this is legal:

player moves commander to cliff edge and slowly builds walls , until the walls knock him into the water.

Is this hacking or legal?+++++

See this replay
#22898519
time stamp: 3:30 - building
4:00 drops

It is considered as exploit.

so by exploit , you mean a hack right ? Does that make it a legal exploit? or an illegal exploit?

@CoffinNoob This exploit is not allowed - Please report it via the report-form as usual.