Let's search the Great Soviet Encyclopedia!

[H. Ross Peron]

On Jesus:

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[H. Ross Peron]

On Jesus:

...

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.

I suspect with the comparative lack of knowledge of ANE religion and society , there was wider acceptance  of the Victorian theories of people like Kelsey Graves (similar to the wider acceptance of Freudian psychology). I'm rather surprised personally at the relatively brevity of the Jesus article-its about the same length of even slightly shorter than the article on Mohammed and that's with half of the Jesus article being devoted to the discussion of the "Christ Myth" theory.