It's an oversimplification to assume the hypothetical Python implementation would have less errors. We don't know that. Some type of math errors would overlap between Excel and Python. Some types of errors would be easier to stumble into in Python than Excel.
For example, an inexperienced programmer in Python might use floating point instead of Decimal data type to add currency amounts. His programming loop to sum the amounts would be incorrect. In MS Excel, adding currency in cells correctly down to the penny is a no brainer. Sure, the Python programmer can be taught "best practices" to avoid this type of error but the point is that while Python helps eliminates some errors, it also creates new ones.
Another example is data munging. A programmer might write some Python to slurp a data file (exported from mainframe or whatever) and do some financial calculation on it. Load the values into an array some other memory structure. The problem is that the memory contents are "hidden" from sight unless the programmer uses a visual debugger or prints out all the values to inspect. The Python programmer may not notice that some values are misaligned or garbage. With Excel, the non-programmer imports the data file and he immediately scrolls through the worksheet as a sanity check. His eyeballs notice that the source data is dirty. Again, the Python "problem" can be corrected with best practices but the point still remains: different tools create different problems.
Lastly, Excel spreadsheets are easily emailed among dozens of people. You can't send ???.py programs to everyone because you can't expect all the Windows users to have the Python runtime. Spreadsheets are even sent to iPhones and tablets and you definitely can't expect easy Python deployment there. With less coworkers examining the ???.py file, it has potential for more errors compared to a scenario where everybody can participate in questioning the spreadsheet's numbers.
Our intuition says that on balance, the hypothetical Python/Haskell solution should have less errors than MS Excel but I can't confidently say I've seen any definitive proof of that.