Start with the vocabulary, because the platform uses it loosely. A co-host is someone given permission on an existing listing to handle messages, calendar, pricing and cleaning. The host of record stays the account the listing lives under, normally the owner. The platform protection programs attach to that listing and that account. Whether a co-host counts as a covered party depends on the current program terms, which the platform rewrites periodically, so read them as they stand rather than trusting a summary.
More useful point: these programs aren't an insurance policy you own. They're a limited promise from a company, with exclusions, claim windows and dollar caps, and no obligation to defend you in a lawsuit. Your own coverage is a separate question. You're running a service business, so most co-hosts carry general liability for their own operations, and some carry errors and omissions for things like mispriced nights or a botched booking. An insurance broker who writes short-term rental accounts is the person to price that, and coverage rules vary by state.
The thing that will decide who pays, though, is the co-hosting agreement, not the platform. Being named in a suit and being liable aren't the same thing, and a plaintiff's lawyer names everyone who touched the property. If your agreement is silent on indemnification, you're arguing from zero. If it says the owner indemnifies you for property conditions and you indemnify him for your own operational mistakes, everyone knows where the dock sits. Have a lawyer in your state look at whatever you sign.