who dismissed it as US propaganda.

Do you have a source on that?

These are the least radicalized radical Islamic terrorists in living memory.

1. They didn't seek to become martyrs, but instead ran away (with a well-planned extraction plan, as far as we can tell) instead of maximizing casualties

2. They hid their faces instead of proudly declaring their martyrdom

3. No screaming of Allah, no mention of Allah, no "God is great", nothing

4. Video shows the terrorists doing Shahada with their left index finger (which goes against the teachings of Muhammad)

‘Abdullah b. Zubair narrated on the authority of his father that when the Messenger of Allah (ﷺ) sat for supplication, i. e. tashahhud (blessing and supplication), he placed his right hand on his right thigh and his left hand on his left thigh, and pointed with his forefinger, and placed his thumb on his (milddle) finger, and covered his knee with the palm of his left hand..

In fact, the controversy surrounding the issue AFAIK is relating to when the finger should be raised and whether you should be allowed to move the finger during the prayer, not which hand (which has been established).