The Microsoft Graph APIs in Microsoft 365/Office 365 give you pretty much all of the Excel execution engine as REST endpoints "in the Cloud" if you just store your Excel files in SharePoint.
It's not surprising the number of turducken business applications being built exactly this way. With Named Cells you don't even have to hard-code cell numbers, just tell them to name them specific things, and Excel users are very happy with the amount of flexibility to rewrite the spreadsheets at will.
It's not necessarily the sanest approach to building software, but no one ever accused most enterprise software development of being sane.
As a rapid prototyping tool, it doesn’t sound terrible, honestly. Many people are comfortable with Excel, so let them use it! You’re gonna use some calculation engine on the backend, might as well be the tool that contains the ”reference” calculations.
It's not surprising the number of turducken business applications being built exactly this way. With Named Cells you don't even have to hard-code cell numbers, just tell them to name them specific things, and Excel users are very happy with the amount of flexibility to rewrite the spreadsheets at will.
It's not necessarily the sanest approach to building software, but no one ever accused most enterprise software development of being sane.