Professor Henry Patterson, himself no friend of Sinn Fein or the Provisional IRA, examines the charge that sectarianism fuelled the IRA’s violence, taking Fermanagh and South Tyrone as an example. You can read the full article here, but here is his conclusion:
‘This article revisits the debate, hosted by this journal in the 1990s, on whether the Provisional IRA campaign was sectarian. In the light of current debates about how Northern Ireland deals with its past, it challenges the analysis that emphasises the non-sectarian ideology of republicanism and ignores the effects of IRA violence. It uses research on the IRA campaign in Fermanagh and south Tyrone to argue that the campaign was unavoidably sectarian but rejects current attempts to label it a form of “ethnic cleansing.”’
The Broken Elbow 
The Provos could dip into blatant sectarianism as and when it suited them, with Kingsmill being the most obvious example. Since the majority of their victims were from the Protestant community their campaign was certainly perceived as sectarian, and perception is everything in this case.
I suppose the most telling question is how many leading members of the Provisional IRA were from a Protestant background?
See the reply and criticism of Patterson’s article by Robert W. White Provisional IRA Attacks on the UDR in Fermanagh and South Tyrone: Implications for the Study of Political Violence and Terrorism
https://www.tandfonline.com/doi/full/10.1080/09546553.2011.561674?src=recsys
The article is available as PDF in IREF archives