Let me start off by saying that I think you have an awesome website and your product is well marketed. That being say, no, I would not use this to teach SQL nor would I recommend it to people learning SQL. And it's not because I think you have a bad product; it's due to the fact that Excel is a spreadsheet management tool, not a database.
I applaud your looking for innovative ways to market your product but this has the potential to do harm to someone's career IMO. I can only imagine a job interviewer's response when someone says they learned SQL using Excel. Excel is not a database and, if you were to try to teach people SQL using a non-database product, I think you are doing them a disservice. Same goes for Google Spreadsheets or any other spreadsheet product. They aren't databases and should not be confused with a real database. IMO "learning SQL" is not the same thing as "learning how to write queries" yet that is what you imply with this tool/question. I think the SQL community across all platforms would not support it for these reasons.
Excel is a great way to learn what a column is and what a row is but it's a horrible tool to learn what a table is, why you need a primary key, what foreign keys are and do, or about the various data types. You cannot learn SQL if you ignore those elements and Excel just does not do those well. Even something as small as MS Access would be a massive improvement over Excel. You don't need an RDBMS for SQL but you do need a DBMS that checks types and does PKs/FKs or what's the point?
My background: I teach SQL Server for my company, http://www.learnitfirst.com/, and have been teaching SQL since the '90s. I think I have about 1600 SQL-based videos online at this time (if not more!).
EDIT: Removed certain items. Email if you want to hear them