Someone can correct me if I'm wrong, but I believe HEV-only cars typically do spurts of EV-only when they can. The difference is that PHEV electric motors are typically larger in addition to their larger traction battery, so they can both go much longer in EV-only mode, and go much faster in EV-only mode.
The HEV only has a bit over 1 kWh of battery, and it also has a lower HP electric motor compared to the PHEV. I can keep my son-in-law's Ioniq HEV (identical powertrain to the Niro) in EV mode on the freeway when there's a slight downhill slope, but in general the engine will fire pretty quickly after pulling away from a stop sign in a residential area (so not accelerating hard). The only place I've been able to drive it solely with EV is in a flat parking lot that I'm not going much more than 5 MPH.
The PHEV has almost 9 kWh in the battery (and getting more for 2023), and a little more powerful EV motor (it also is getting larger for '23), so it's capable of pure EV mode up to freeway speeds without an issue. You just have to keep the throttle foot light on the pedal, as there's a limit you pass that will still kick the ICE on if you press too hard. And on steep hills it can be next to impossible to not use the engine. With the improvements in the upcoming '23 PHEV, I might switch back from pure EV to a PHEV. Have to see what they're like in person.
