I think that both you and the article are actually underselling the problem. The problem is not that terrorism has slanted towards right-wing extremism, so much as that terrorism is still a right-wing extremist tactic, and the overall amount of terrorist attacks have increased.

The most famous terror attacks in the USA [0][1][2][3][4][5] were perpetrated by right-wing folks: These were religiously-motivated, racially-motivated, in protest of large and overly anxious government, and stand in stark contrast to ELF blowing up SUVs late at night.

WP has a list of terror attacks in the USA [6], if you want to peruse a complete list and see for yourself how often people are motivated by religion, race, or other hateful right-wing delusions.

Which events would you say that the original article has gotten wrong? How is it "convenient" that terrorism is being committed in a lopsided way?

[0] https://en.wikipedia.org/wiki/1984_Rajneeshee_bioterror_atta...

[1] https://en.wikipedia.org/wiki/Ted_Kaczynski

[2] https://en.wikipedia.org/wiki/Oklahoma_City_bombing

[3] https://en.wikipedia.org/wiki/September_11_attacks

[4] https://en.wikipedia.org/wiki/Boston_Marathon_bombing

[5] https://en.wikipedia.org/wiki/Orlando_nightclub_shooting

[6] https://en.wikipedia.org/wiki/Terrorism_in_the_United_States