Do not be overly literal with short messages on a website or mobile app. The term "VISA" is simply a generic reference to evidence that one is permitted entry, in this case, to the US. While an ESTA is not a visa, if one has to choose a single word which flags the need for a document check, it is this.

Not only does DL need to verify return plans because of the one-way ticket, but it also randomly kicks out OLCI for further document checks, as do most carriers, including for US nationals traveling to other countries.