So Steemit is a cross-platform client for the IPFS protocol similar to what qBittorrent is to the BitTorrent protocol?

From what I'm able to tell, yes, especially the sites that provide uploads for big content like videos (dtube).

isn't Po.et just a canonical tag put on a blockchain?

Yes and no, po.et also allows for licencing per post to allow for reuse, otherwise the default on Steemit is that all content is copyrighted by their respective authors with distribution rights reserved unless stated otherwise explicitly on the posts in question or on their profile.