inverter specifications? 85901A AC Power Source vs APC

[CopperCone]

I actually own a 85901A AC Power Source.

The specification for the waveform is

 Voltage135 or 270 peak ± 5% rectangularwaveform with 25% dead zone(115 or 230 Vrms ± 5%)

How does it compare to something like a APC backup battery? I'm wondering if there is anything special about this inverter.

https://doc.xdevs.com/doc/HP_Agilent_Keysight/HP%2085901A%20Operation%20%26%20Service.pdf
Page 81 has a picture of the output waveform and the prior pages have some description on the control circuit.

It also claims to comply to a VDE0871 level B EMI standard.

How well does this thing compare to computer backup batteries? Like a APC2000

https://www.hardwareinsights.com/database-of-ups-output-waveforms/2/

I imagine the sine-wave output would be superior.. but I don't know much about this stuff. Any reason why they have such a seemingly crappy output waveform on something meant to plug into sensitive spectrum analyzers?