The news said all 4 employees were Singaporeans.
No. what Nikkei is saying is:
1. they have not independently verified the nationalities of the four ex-employees but has been informed by sources or contextual clues.
2. some level of uncertainty ("understands to all be Singaporeans"), it leaves open the possibility that later verification could prove otherwise.
Actually hor, if Nikkei phrase their statement like this, it could also be that the mention of their Singaporean nationality implies that it is relevant to the context of the discussion, possibly in relation to the nature of the case, legal jurisdiction, or public interest.
(With help from ChatGPT)