Does Xfinity Home Phone service provide a true landline?

Their home phone service is VOIP between the Internet/Wifi/Voice gateway device in your home and Comcast (usually over a combination of coaxial cable and fiber optics), but the phone jacks on that device provide a POTS (analog "Plain Old Telephone Service") interface to customer touch-tone telephones, not VOIP.

... it's unclear from my research if these only provide VOIP service or if I can connect them into the wall and activate the rest of the wall jacks. ...

If you first make sure the wall jacks are not already connected to a previous provider's network, connecting a modular base cord between the L1 gateway jack and a wall jack should, if the wiring was done in the usual way, energize the remaining jacks in the home. Whether that constitutes a "true landline" is a matter of interpretation I'll leave up to you.

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Hello @kpc3, I see BruceW provided some helpful information about this concern. If the wall jacks are not connected to a previous provider's network, connecting a modular base cord between the L1 gateway jack and a wall jack should work.