You seem to misunderstand, the book is in fact quite unrelated to the HG Wells book apart from the name, they just took the same theme (Martians invading earth) and put a completely different spin on it. It's not a follow up, and I for one wouldn't go as far as say that HG wrote the "definite" book on all Martian invasions. There are plenty of other themes to explore within that context. The Strugatsky brothers, being great satiricists with a good sense of humour, made a joke with their allusion to the HG book (also, the agenda of the Martians remain unknown all throughout the Strugatsky book, there really aren't any conclusions to be drawn on that area). It was a joke, although personally I find the Strugatsky book to be a lot more interesting both technically and on a conceptual level.