They’ve done a similar thing at a smaller scale with individual phones in the past. What is different is this time it’s not targeted at a specific person and instead involves thousands of devices going off simultaneously. It’s not a big risk unless you have nation state level threats up against you because it’s hard to pull off, they have to get a functioning device with explosives in it into the hands of the target and the effort involved in doing that is significant.
Almost certainly it was explosives. Mossad very likely designed a functioning pager that contains explosives but looks identical to the original pagers and this is effectively a supply chain attack.