Reading through reporting mainly via Twitter it appears most have accepted the claim of responsibility made by ISIS. Oddly IMHO this BBC News report is less convinced:
In the past, IS has sometimes claimed attacks that it was not involved in or which it simply inspired. But the details from IS would seem to back up the government's assessment.The choice of targets is much more in line with IS ideology than with the traditional types of communal violence seen in Sri Lanka.
A comment with far wider significance, with my emphasis in bold:
There are still questions - did the local men affiliate themselves to IS or receive direct support? Did they travel to Syria or to other countries? The Sri Lankan government has said it believes some of them had spent time abroad, but how significant was that to the plot?

Answering questions like these will be important not just for Sri Lanka but other countries as they try and understand whether other relatively small, locally focused groups could be capable of transforming a threat into violence on such a massive scale.
Link:https://www.bbc.co.uk/news/world-asia-48028045


Then there are two US-based SME commentaries. From CFR a short Q&A 'Sri Lanka Bombings: What We Know?':https://www.cfr.org/article/sri-lank...s-what-we-know

A more comprehensive 'Lawfare' article by Daniel Byman:https://www.lawfareblog.com/attacks-...reign-fighters