Gotcha, my mistake. However, I've read the study and has some serious flaws in my opinion. Even politifcat which tends to be liberal rated the claim as only half-true.
Professor Andrew Holt (Florida State College) explained it much better than I can so I'll just put his quote down below and bold out the important parts.
The problem with this source, as I see it, is that the count is wrong.
In International Security’s listing of the 45 deaths due to Islamic extremism, they attribute them to only nine incidents since 9/11. These include the more well-known attacks, such as San Bernardino (14 dead), Chattanooga (5 dead), Fort Hood (13 dead), the Boston Marathon Bombing (4 dead- with 264 additional casualties, I might add), as well as the Washington and New Jersey killing spree (4 dead), but also the Oklahoma beheading of 2014 (1 dead), the Little Rock Shooting of 2009 (1 dead), the Seattle Jewish Federation Shooting of 2006 (1 dead), and the Los Angeles Airport shooting of 2002 (2 dead).
So this is where they stop, but if we are comparing Islamic extremism to right wing extremism, apples to apples (and, to give credit to International Security, they acknowledge this is subjective on their website) then there are several others incidents that should be included in this total.
Basically a lot of events that did occur in the Islam weren't even included in the study. Basically many lone wolf attacks or self-radicalized Muslim terrorism incidents weren't even included
In June of 2006 in Denver, a man shot four of his co-workers and a swat team member, killing one. He later claimed he did it because it was “Allah’s choice.” In December of 2009 in Binghamton, a Saudi Arabian graduate student named Abdulsalam S. al-Zahrani killed Richard T. Antoun, a non-Muslim Islamic studies professor who served on al-Zahrani’s dissertation committee, in revenge for “persecuted” Muslims. Prior to the killing one of al-Zahrani’s roommates tried to warn the university administration that he had been acting “like a terrorist.” In 2012 in Houston, in two separate incidents in January and in November, two people were shot to death by a Muslim extremist for their roles in his daughter’s conversion to Christianity. In March of 2013 in Ashtabula (Ohio), a Muslim convert walked into a Christian Church during an Easter service and killed his father, claiming it was “the will of Allah.” In August of 2014 in Richmond (California) killed an Ace Hardware employee by stabbing him seventeen times, claiming he was on a “mission from Allah.”
And to conclude the professor also stated:
Right wing terrorism is more deadly for Americans only if you add a number of very limiting parameters (e.g. excluding the victims of 9/11, ignoring “lone wolf” attacks without solid connections to groups like al-Qaeda and their affiliates, etc…). But if you lift those limitations, and apply equal standards, then the raw and unfiltered numbers of deaths of Americans due to Islamic extremism in the United States over the last fifteen years dwarf the numbers attributable to right wing extremism by a ratio of over 62 to 1.
Even if you leave out 9/11 victims and just focus on the ideological statements and goals of the attackers, then the deaths of Americans due to Islamic extremism still outnumber the deaths attributable to right wingers (which reveals an even greater disparity when compared with population groups). If we move beyond America’s borders, then the disparity becomes far greater, with somewhere around 90% of the world’s terrorism related deaths attributable to Islamic extremism, and only a fraction of 1% attributable to right wing extremism.
So there you have it. The study had a loose definition for Right wing extremism while a very rigid idea of what an Islamic extremist (Apparently saying allahu akbar wasn't enough to indicate your motives, who knew? Guess he must've just been frustrated with the long line at Chipotle!)