The 21" they are talking about is the slightly sloping TT of most mountain bikes of 10 years ago. The steeply sloping top tubes of modern hybrids aren't measured on the same scale. You can find seat posts in nearly any length now as you already know. When you put your elbow at the tip of the saddle and reach your fingers to the handlebars do they reach? I'm happy when my fingers are just short of the bars on a flatbar and when they actually overlap by about a half for a drop-bar situation. Your bike appears to be using modern threadless stems which are cheap and easy to replace. You can get any length you want. Sadlle height isn't arbitrary. There are guidelines for setting it. You were possibly riding around with your saddle way too low for a long time or you now have it way too high. Why guess? Good luck.

H