Correct me if I'm wrong, but it sound like Free/Basic groups are not able to
I'm not sure where you got that impression. Mark said: "Free/Basic groups can allow sponsorships."
He does go on to say that:"Converting from Free/Basic to a Premium group still requires a credit card to be on file when you do it, [emphasis mine] but you can delete the credit card after you've upgraded the group, relying only on sponsorship reserves to pay for premium." I think that applies at a later time, if and when the group owner decides to upgrade to a premium plan, which might not be until the group has accumulated enough in their sponsorship reserve to pay for the upgrade.
My hope was that there would also be a method to somehow allow funds to be collected from members to help support the Groups.io service without having to upgrade a Free/Basic group ...
I think that's what happens if the group owner never applies the funds to an upgrade. Groups.io has the desired support and the group owner didn't need to even temporarily provide card information.