It's indicated in the datasheet that the device is an IEC Class II device which means a device having a "double" insulation.

If this is not satisfactory for you, double insulation requires that the device should withstand at least 3 kVac, and the datasheet indicates that the withstand voltage is 4 kVac.

So no, you don't need an extra insulation transformer.

The entire question is under the scope of IEC 62368-1.

To the 1st question:

It's indicated in the datasheet that the device is an IEC Class II device which means a device having a "double" insulation. Double insulation requires that the device should withstand at least 3 kVac across its input and output, and the datasheet indicates that the withstand voltage is 4 kVac.

To the 2nd question:

By placing the power supply and the circuitry supplied from it into a metal enclosure and ground the enclosure by bonding the PE cable, you make the entire system Class I, which means a basic insulation as well as safety ground.

In either case, you don't need an extra isolation transformer provided that the end-user will never have a chance to access the live voltage "inside" or "around" the product during normal use.