A promising start.
The border is not 100% secure to Covid in the same way that our border is not 100% secure to anything. This is why the testing takes place at entry and in MIQ.
Testing prior to entry would require a secure post-test accommodation. Testing at early infection is nowhere near 100% reliable, so the reality is you would need 2 tests, about three days apart prior to boarding to be around 95% certain (but not 100%) that the traveller was not positive.
Good luck managing that in ports outside your own country.
Of course, if MIQ was the one and only me