I have this inverter but I didn’t measure the 3 phase output voltage as it seemed to work just fine. It’s interesting that 240 x sqrt(2) is 157, which is a guide between peak and rms. But of course the 240V input is already the rms so I can’t see why it wouldn’t be 240V on the 3 phases output. Hopefully someone closer to 3 phase could confirm this is what you should see.
The other possibility is a faulty unit of course. If you get confirmation that it should be 240V then let the seller know and see what happens.