That's the first I've heard of hollyhocks being biennial, though I can't say I've done a lot of research. I only know what I've experienced with mine. As, I've mentioned, I have miniature. A neighbor has regular sized hollyhocks. Both come up and bloom every year.
Could be they act differently with maturity or various climates? I've had mine, I'm not sure, but at least 4 years. My neightbor has had hers longer than that. This year was by far the best mine performed. With more stalks, more flowers and longer blooming, it was a banner year.
Wendy said she had a good year too. Sounds like you did as well with 5'! I have theorized with mine it was either the summer conditions (cooler) this year or the fact that they have matured. Or both. I'm curious what will happen next year.