On the Singapore Visa Application page, under important notes is the following statement:
A Singapore visa is not an immigration pass. It is a pre-entry permission for the holder of a valid Singapore visa to travel to, and seek entry, into Singapore. The grant of an immigration pass will be determined by the Immigration & Checkpoints Authority (ICA) officers at the point of entry. Possession of a valid visa alone does not guarantee entry into Singapore.
This would seem to indicate that a denied visa is not a refused entry. After all, they are saying their visa is simply a means to travel to Singapore and seek entry into it.
Contacting the consulate for your country would, of course, be preferable.
Also, remember that they are asking if you have ever been deported from or refused entry to any other country as well. Other countries may consider a visa refusal/denial as refused entry.