I looked up "Валентина Сынах" Skype user and, as expected, their full phone number is +7904xxxxxxx (see full number in the article), with Russian prefix +7. Note, WHOIS records have US number +1904xxxxxxx. Not sure why the author only searched for the last 10 digits.
+7 904 code is a very busy area code in Russia shared by several major carriers, covering the majority of Russian regions, so the probability of someone having any particular 7 digits in that area is pretty high. For example there's someone else registered in Skype with number +7904xxxxxx4, just one digit apart. I haven't been able to find an English source, but you can look at the list of carriers and regions in Russian in [1].
Also, US area code +1 904 matches the address in WHOIS records (both Jacksonville, FL), so at least it doesn't look like someone just changed the first digit and used their own Russian number.