Yup, and the OP has identified one of the limitations with it… just how hard are you supposed to go during the 5 min “blowout” effort?
That said, as I have always emphasized, I am not a coach, much less one who has to deal with clients remotely. As well, although it isn’t the most precise approach, on average taking 95% (vs., say, 93%, or 97%) of 20 min power seems accurate (just not sufficiently precise, at least IMHO). However, it isn’t the definition of FTP, which is what almost every scientific article on the subject gets wrong.