To clarify it was this statement in the 2019 review that I was interested in:
"And the gasoline engine now turns on to add heat to the cabin, instead of exhausting the electric battery pack"
Never trust a review to contain factual information. The reviewer did the best that they could with limited time in the driver's seat.
The Volt has always had a standard feature called Engine Assist Heat. Engine Assist Heat only operates when the Volt is running in Normal, Sport Mode or possibly in Mountain Mode. When the Volt is in Hold Mode the ICE is already running, at least most of the time, so Engine Assist Heat would not apply. There is a separate, related, setting for Engine Assist Heat (when) Plugged In that can be set to "Off". This is important consideration if you park your Volt inside a garage, especially if the garage is attached to your home. You definitely want to set Engine Assist Heat Plugged In to "Off" if you normally park your Volt inside a garage to avoid the potential build up of deadly carbon monoxide fumes while the ICE is running during the Volt's 10 or 20 minute preconditioning cycle. Of course if you park your Volt outside you may want the ICE to start during preconditioning to warm the cabin faster.
Engine Assist Heat is designed to use a small amount of gasoline to heat the cabin for the comfort of the passengers is cold weather. When Engine Assist Heat is active the message "Engine Running Due To Temperature", (ERDTT) will briefly appear on the driver information console. The ICE will start and will continue to run, even when the Volt is stopped at a traffic signal, until the engine coolant temperature reaches ~160F. After that the ICE will start as needed to keep the engine coolant between 120F and 140F so that there is sufficient waste heat to warm the air going through the cabin heat exchanger. This strategy works well, conserves the Volt's traction battery charge for propulsion instead of generating cabin heat. The Volt will continue to use some of the battery charge to warm the traction battery as required. Engine Assist Heat only heats the passenger cabin.
You can't turn Engine Assist Heat completely off but you can set it from "On" to "Deferred" In the standard "On" setting Engine Assist Heat will start the ICE when the outside air temperature reaches 35F (1.6C) or lower. In recognition that sometimes the driver will not want Engine Assist Heat to start the ICE, when set to "Deferred", the ICE will not start until the outside temperature drops to 15F (-9.4C) or below. In the 2019 Volt this has been changed to allow for deferring engine assist cabin heating until the outside temperature drops to -14F (-25.5C) or below. (I don't know if the new -14F (-25.5C) deferred setting replaces the older 15F (-9.4C) deferred temperature setting or adds a second lower temperature deferred setting.)