IMHO the trick is that any moderately useful and modern VBA is likely to go way beyond the base specification.

- UserForms GUI (which this does not touch on at all)

- Microsoft libraries like mscorlib and Microsoft Scripting Runtime, to get stuff like ArrayList and FileSystemObject.

- Excel VBA is kind of notorious for mixing Excel code with the VBA, using cells of the sheet as data storage, using formulas inside of cells to calculate values, etc. For example to store a list of values and sort it, they might put the data into a tab of the spreadsheet and call Excel functions to sort it inside the spreadsheet instead of building a list in VBA and sorting it in VBA.

So I think an open source useful VBA clone would probably also need to be an Excel clone and also a clone of a bunch of Microsoft libraries