I think you were influenced by the multimeters, which is why you wrote that you felt that a device undergoing a high voltage test could no longer be sold to the customer.
For electrical appliances and power electronics, the standards are different and this has nothing to do with multimeters.
There are several classes of isolation, I refer you to Wikipedia for more explanations.
https://en.wikipedia.org/wiki/Appliance_classesChargers and power supplies belong to Class II double insulation.
Since the secondary is not grounded, the safety of the user depends only on the quality of the insulation between primary and secondary.
This safety is guaranteed by an Hi-Pot test.
The test conditions are difficult to find on the Internet because the standards are protected by Copyright and are not free available on the Internet .... Moreover, these standards are complicated and difficult to interpret correctly.
As far as I can remember, the test voltage applied for Class I equipment is 1500Vrms in 50 or 60Hz or 2121Vdc and 3000Veff 50 or 60Hz or 4240Vdc for Class II.
The test time is normally 1 min. But can be reduced to minimum 1s. (It should not be possible to make a test of 1 min. duration in mass producted appliances.)
It is intended that the voltage should be applied gradually. (ramp).
This test is fundamental and it is the only guarantee of safety for the user against the risks of electrocution.
It should be remembered that in Europe, many distribution networks have a 230V ground-phase voltage and the risks are high.
Is this test actually done at the factory?
There is no guarantee and no way to verify it.
So you can only trust in the well known brands.
Any no-brand or unknown brand, or fake gear is therefore not safe.
To answer Charkel, no, this charger is not safe