Every public spreadsheets (blockchain or not) can be used as a security exchange, imho.
Every exchanges / marketplaces are basically just a "place" for two parties meet, agree, and record a transaction. Now, two parties can also meet, agree, and record a transaction on Google Sheet, Apple's Numbers, Microsoft Excel, or any kind of spreadsheet. Although the softwares and the providers are not legally stated as exchanges, however, functionally, they can act as exchanges.