Not only is it likely that, as you say, Rob Pike was aware of sum types, but he did not create Go on his own but it was created by a small team. Someone like Robert Griesemer, who studied under Wirth, would have known about them, if the others hadn't.
I have been using the Wirth languages a lot (Pascal, Modula), and one of the big appeals of Go to me is, that it brings back a lot from those languages to the modern times. The Wirth languages are far to underrated in programming today.
The irony being that most Wirth languages are more expressive than Go will ever be, with the exception of the first release of Pascal and Oberon versions, and the follow up on minimalist design approach with Oberon-07.
When Go came out, I though it could follow Oberon, starting small and eventually reach Active Oberon/Zonnon expressiveness, but alas that is not how they see it.
I have been using the Wirth languages a lot (Pascal, Modula), and one of the big appeals of Go to me is, that it brings back a lot from those languages to the modern times. The Wirth languages are far to underrated in programming today.