No, it’s absolutely not true. 99.7% of adult Russians can read and write in Russian. There is a tiny minority of people who struggle to read and write in Russian, because they almost exclusively use their own language. Of course, being able to write in understandable Russian is not the same as being a perfect speller. While Russian spelling is easier than the English one, there are plenty of traps for the person who doesn’t write a lot and/or doesn’t spell check.