comparing estimated deaths over 525 years to deaths that occurred over less than 100 years is orders of magnitude worse than comparing apples to oranges.
comparing estimated deaths over 1,400 years to deaths that occurred over less than 100 years is so stupid it's disgusting.
This site: Twentieth Century Atlas - Death Tolls
has some very interesting, detailed, and macabre, statistics on 20th Century megadeaths. Note that the linked page only counts events in which 3 million or more people were killed.
Stalin's death toll - exclusive of war deaths - is about 30 million, over the course of 29 years.
Mao's death toll - post WWII - is about 40 million, over the course of 26 years.
WWI death toll is about 15 million, over the course of 4 years.
Russian Civil War death toll is about 9 million, over the course of 5 years.
WWII death toll is about 66 million, over the course of 6 years.
Adding all of those up, there were a total of about 160 million deaths during the Twentieth Century attributable to megadeath events such as wars or totalitarian regimes. In fact, since these figures come from the period 1914 to 1975, these 160 million deaths took place over about 61 years.
Note that Hitler's deaths were included in the deaths for WWII; however, the website breaks out deaths attributable to Hitler of approximately 15.5 million (not including deaths attributable to WWII itself).
In order to make the comparison closer to comparing apples to apples and oranges to oranges, it might help to compute the per-year death tolls.
Per-year 20th Century deaths from all megadeath events: 2,622,951
Per-year deaths attributable to Stalin: 1,034,483
Per-year deaths attributable to Mao: 1,538,462
Per-year deaths attributable to Hitler - assuming the deaths took place during Hitler's entire reign from 1934 to 1945 (11 years): 1,409,091
Per-year deaths attributable to all major Twentieth Century political ideologies (excluding those attributable to the world wars) - 85.5 million (Stalin, Mao and Hitler) over the course of 51 years (from the start of Stalin's regime to the end of Mao's regime, and not double-counting years that overlapped for one or more of the totalitarians): 1,676,471
By comparison, using the estimates provided in the article, there were 80 million deaths in India over the course of 525 years, or about 152,381 per year.
Also, again using the article's estimates, there were 140 million deaths attributable to the slave trade over 1,400 years, or about 100,000 a year.
When these megadeaths are compared using the same metric - deaths per year - those the article attributes to muslims pale into insignificance next to the deaths attributable to just the three major totalitarian political ideologies of the 20th Century.
deaths in the hundreds of thousands a year attributable to ideology cannot simply be dismissed out of hand, but the muslims were pikers compared to the likes of Stalin, Hitler and Mao who, collectively killed more than 10 times per year the number of people the muslims killed per year.
Something else to keep in mind is that the deaths attributable to muslims are not broken out and attributed to individual leaders, whereas those attributable to Stalin, Hitler and Mao can definitely be laid at their feet - the deaths attributable to them are really just that - attributable to them - in a way that the deaths laid at the feet of the muslims cannot be laid at the feet of particular individuals nor their cults of personality.
Compared using the same metric, the single worst killer in human history is Mao. Not Stalin, not hitler, not the muslims.