So if, when Standard Oil was still operating as a monopoly, a single well had decided to refuse to sell to them - or even a dozen wells - or a conglomerate of wells - that means they'd no longer be a monopoly?

Just because you don't own 100% of a market doesn't mean you don't have a commanding position.

1) Comparing Tik Tok to a single oil well is laughable.

2) If someone who owned a single oil well in time of Standard Oil could get their oil to consumers and profitably sell it, then however big Standard Oil might be, then it didn't have a standard monopoly. Of course, that's not how things work back then. Standard Oil controlled the refineries and so well owners were over a barrel. Facebook has tried to control the Internet pipes but basically failed everywhere.