Yes I noticed the requirements after I purchased the tooltip extension. I do have to say that I am happy that the extension does in fact work with Dreamweaver CS3, you just do not get any preview when working through the interface. The end result though is what makes me happy.
As you can see in the screenshots below, the extension does insert the correct code, and the validation tooltips do in fact work, again as shown in the below screenshots.
So, as long as you are not going to change anything with the extension, "I should have added a restriction in the MXP installer to make sure that it couldn't be installed into earlier versions of Dreamweaver." even in later releases, I am inclined to keep it because for me it suits me just fine, and the price was right.
So, please don't include anything in future releases so this extension cannot be installed in earlier releases of Dreamweaver than version CS4.