More-or-less. LibraryH3lp is a flat-fee based on population served, but that is for a top-level administrative account.
In practice, our permissions system lets you do account sharing that is almost, but not completely, transparent. If that works for your needs and you're serving the same population, then yes you can treat it like a site license.
If you really need multiple top-level accounts, they will each need a separate subscription. Additional subscriptions beyond the initial subscription are available at a reduced rate if the initial subscription is higher than our lowest subscription rate.