You are accurate, based on what I've read, on the religious make up of Sri Lanka.
All of the interviews I've heard from various high ranking officials and their (and the Government's) approach have been highly commendable and measured in such tragic and difficult circumstances.
However, they have apparently arrested 24 people, some appear to be foreign nationals and I've heard one unsubstantiated report of a Dutch person being involved, along with another unsubstantiated report that apparently an Islamic Jihadist group had warned of pending attacks.
Possibly a load of rubbish, possibly the truth.
The Churches, congregations and the timing suggest that it was religiously motivated and it is highly unlikely to have been Buddhist or Hindu inspired.
The motive for the attacks on the hotels is less clear.