In What is the difference between do, tiel, tial, tiom, and sekve? the answers address the differences between all the title’s words—except for do.

I just saw a Duolingo sentence,

Mi ne ankoraŭ aliĝis, do mi ne estas aliĝinto.

and Duolingo didn’t accept tial (for that reason) as a substitute for do, and I can’t find a resource that explains. (PIV’s entries on do and tial make it seem like tial/do should frequently be substitutable.)