Electrician doesn't have to do anything because they sell cord adapters with the phases bridged.
However, doing this (running both legs of a 240V system off of a single leg 120V system is kind of kludgy. For the same money as that Honda you could get a larger 240V inverter gen with a proper L14-30 240 outlet. These will also make more power (usually 240V inverters start at around 5000W) so you'll be able to run more things. Suggest you get one with dual fuel.
Like this:
https://www.amazon.com/GENMAX-Generator%EF%BC%8CGas-Compliant-Lightweight-GM5500i/dp/B09GVFWF1F
Hondas are great but they are really TOO good for occasional backup use. For the price of the Honda you can get 2 or 3 perfectly adequate gens.
Even without eco throttle, an inverter gen is going to use less fuel than a synchronous gen.