Unfortunately I don't have access to the book being reviewed. Nor do I know Polish, so I couldn't read it even if I did. So that's all I can find out about this Polish vocabulary hoax. (Though it sounds more like an urban legend than a hoax.) Googling '30,000 new words in Polish since 1945' doesn't turn up anything helpful either.
However, a January 2013 article in The Independent notes that Polish is now the second most widely spoken language in England and Wales, after English itself. And that this Polish-English contact is having a large influence on the Polish language, especially Polish business speak, which is adopting numerous English terms:
She added: "Marketing is often referred to as 'marketing' and you would also say 'IT' rather than the 'technologia informacyjna' or 'TI' either. People tend to use these English words whether they fully understand what they mean in English or not. PR is also Polonised so it is pronounced like the English 'PR' but spelt in Polish to reflect the pronunciation 'piar'.
Based on this, I would imagine that it must be true that there have been many new words added to Polish since 1945. But apparently not 30,000 new words (and 1 million technical terms).