was Mythicism more popular in 70s scholarship or are they (presumably) intentionally overstating its influence?
If I recall right Engels tended to treat Jesus more as a mythological figure than as someone who really existed, so Soviet authors followed him. Not an actual answer to your question but yeah.
though Kautsky did endorse the categorical idea of a historical Jesus, though he denies the historicity of the vast majority of what's in the Gospels, including events that are seen as certainly historical by >98% of contemporary scholars. Kautsky was, of course, force-fitting Jesus into a model of anti-imperialist revolutionary and nothing more.
Kautsky was probably treated as an unperson a la Trotsky in official Soviet History, but he's another data point on how Marxists treated Jesus.