That’s not accurate. Anthropogenic GHG emissions certainly weren’t nearly as large in the early 20th century as they are now, but they weren’t zero, either. The early 20th century warming was partly ‘natural’ (most of ‘natural’ being solar), and partly anthropogenic.
The warming over the last half-century has been almost entirely anthropogenic. Is there a possibility that the majority of this warming was from a non-anthropogenic source? Sure, but the probability is extremely slim. Realistically, well under 5%.