You're both right. Sounds like a law that hasn't been updated yet for online ads.

>The laws that prohibit foreign nationals from spending money to influence U.S. elections do not prevent them from lawfully buying some kinds of political ads on Facebook and other online networks

While in one sense true, the Fortune article seems to be reading the word “television” (or perhaps “audio or audiovisual content”) into the phrase “cable, broadcast, or satellite” as a distribution mechanism when it is not, in fact, present where that phrase is used in FECA, as amended (FECA does have radio and TV-specific provisions, and they expressly name radio and TV, and they aren't the ones of interest here.) Absent case law to the contrary, which is not cited, it would seem by it's plain language to cover most real electronic media including the internet (advertising on a isolated RFC 1149 network would not be covered, of course, nor would ads in plenty of pre-telegraph “old media” like dead-tree-only newspapers.)

You can take your assertions further -- every Russian, national or not, is an agent of the Russian government with 101% certainty at that. Furthermore, everyone who associates with Russians is an agent as well, with a reasonably high degree of confidence.