Well, he built the system, Emacs is the front end. It's just an awkward front end for most of the people, but with proper training, anyone can pick it up, it's not like ask them to maintain the system.
For new formulas, yeah, It's hard to input new formulas, even in spreadsheet. The system is somehow complex, you probably need a UI for new formulas too, even if it's in spreadsheet.
Honestly, I'm not so sure. Emacs isn't known to be user friendly, I can easily imagine many non-techies simply refusing (consciously or, more likely, unconsciously..) to learn it.
Turn on CUA mode to get the traditional shortcuts, don't hide the menu bar and toolbar - and now Emacs is being operated exactly the same as any other programmer's text editor out there.
The reputation of user unfriendliness is undue, and based mostly on looking at how pros work with it.
For new formulas, yeah, It's hard to input new formulas, even in spreadsheet. The system is somehow complex, you probably need a UI for new formulas too, even if it's in spreadsheet.