My kids do this all the time. They also use the touchscreen in conventional laptop configuration (not folded-flat tablet mode) on their Chromebooks all the time. It's bizarre to me. I'm always trying to get them to use the keyboard, but they don't care. Example: enter password on the keyboard, then tap the log in button on the screen with their finger, rather than just pressing enter. ¯\_(ツ)_/¯
I do this on my iPad with Magic keyboard and I'm a die hard command line user otherwise.
I think the reason I started doing it on the iPad is that the keyboard focus is sometimes inconsistent, so clicking or tab-tab-tab-enter is slower and less reliable vs. just touching the screen. Definitely feel the gorilla arm though.
My 3 year-old never had access to tablets or phones but sees us using phones all the time. So when he gets curious and touches the laptop screen (something he does with our phones occasionally as well), he's shocked to see it doesn't react.