The article doesn’t go into it, but a key advantage they have is that heat pumps move heat, rather then trying to generate it. So they can move a lot more heat into your house than would be generated by running the electricity they use through a resistor. This makes them effectively more than 100% efficient (the exact amount depends on temperature) as compared with burning a fuel or resistive heat.
Can you convert an existing AC system to do both and save money on getting a whole new system installed?
It’s probably not worth it. If you have a system more than 10 years old, then you’re probably going to have to replace the accumulator as well if it uses a different kind of refrigerant. It’s likely cheaper to buy a whole unit and furnace than messing with it.