After reading what you said again, I see what you are saying; It is not a true 2-5-1, because of the Sub. I'm not saying it is a true 2-5-1, but consider this, your playing with a bass player, he/she is watching while you sub with a Db7 chord, they do not see the normal 2-5-1, they see exactly what you are playing for the bass 2-2b-1, however it is the same progression, just with a substitution chord. They can play either the Db or the G in the bass and they will both fit. All I'm saying is that you may not always see the 2-5-1 just by looking at the bass-line. Try the example with both the Db or the G in bass, it will sound good either way, the bass is interchangeable as well as the chords, but the only way to recognize totally what is being done with the sub is to know the tritone chord sub.