On a technicality (which is what trademark attorneys deal in), your point falls to the same fallacy. The product Adblock is an "Advertisement blocking engine" or the like. "Adblock" is a made-up word and therefore likely eligible for trademark. (Or it was before the namespace got polluted with all the variants.)
"made up word" is overly generous. It's the word "ad" followed by the word "block". "ad block" is a valid description of what it does, equivalent to "advertisement blocking". You only put the word "engine" in there to fake there being a difference. Microsoft wouldn't be able to get a trademark on "Operatingsystem".
especially in the light of how, some years ago, their lawyers swooped down like hawks on a Canadian named Mike Rowe who put up a "Mike Rowe Soft" consulting business and web site.
"AbiWord" is a single word, which may be a factor. They might have trouble if it were "Abi Word" or "Word by Abi" or even "Word". It's implausible that another entity could market a competitor of Word (or Windows) by naming it e.g. "Apple Word" or "Apple Windows". I'm going to bet that all the common permutations are trademarked by MSFT.
Also, I'm guessing Abi's limited reach is a factor. If Abi was suddenly taking a meaningful share of usage, they would probably get an angry lawyer letter.
