Actually, the more I think about this, the more I think you are right. If I use the "Excel File" option instead of "Excel Instance", these fields are sent with string formatting. In the Excel Instance export format, we have to send everything to Excel as a variant. Then Excel decides how to format it based on content (unless of course, we send the data with a quote before it, to tell Excel "this is always a string".
The green marker that Excel adds is only present when you have numeric-looking data that has the quote before it.
The only downside I see of sending string fields as strings, is that it prevents you from formatting them as numbers (or dates, or whatever) in Excel later if you want to. And maybe for that, there is some way of selecting multiple values and removing the quote. Not sure about that.
I think your request makes sense. I just want to be very careful with changing it, as Export Dataset is a very highly used feature in Toad.