I'm not sure we've absolutely ruled out that planets finishing the bulk of their formation after a star ignites, and the precursors could be in place by then and we really don't know much, especially in light of this, to discuss the surface or near surface conditions of a world at that time. More, in DNA-based life a thousand generations can produce some serious changes and a generation hardly needs to be even a year. Something built along a different template might have rapidly faster changes and need a far shorter turnover period, so you're absolutely right that we have to be careful extrapolating, but I do feel on general principle that if one sees a Dyson forming around a young star its more likely to have been picked because it was new and young by someone who wanted to get their total money's worth then by a rapidly emerging race, it wouldn't even seem too probable there were any planets of size there and surely they'd have been getting raked over by impacts.