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See, the Darius arcade cabinet had six speakers (also headphone jacks, lmao) placed in and beneath its seats playing stereo audio, and thanks to that, but primarily its triple-wide monitor, it was considered an impressive game for the time. Naturally, it was a prime candidate for NEC's campaign to port popular arcade and PC (pasokon) games to the PC Engine CD-ROM² to show off the advantages of CD, and given its impressive sound setup, they would have wanted a similar edge for their home version- hence the designation of Dolby Surround. (This was also, less prominently, advertised for the port of Taito's Rainbow Islands around the same time and... frankly I'm not sure why, other than the game being known for its (plagiarized) soundtrack?)
Does it actually have it? Well, Dolby seems to think so. See, Dolby Surround (also known as Dolby Pro Logic) is a way for two physical channels of audio to deliver four (or five) source channels to five (or six) speakers that traditionally needs a home theater A/V receiver- left, center, right, and surround (or surround left and surround right- not to mention the subwoofer channel that's mixed from these), as well as modulate their volume. Surround speakers are designed to create sound the reverberates through the space rather than having a clear source.
Although NEC would commission new CD soundtracks for many of its ports, Darius was not one of those games, and the original audio was all chip-generated for stereo, so I have to imagine that, to simulate surround without any decoding to split two audio channels into four, it played "center" sounds over L+R, and "surround" sounds over L-R, as in, each channel half a wavelength out of sync with the other, which is what Dolby Surround does to separate the surround left and right speakers' outputs anyway. Darius has this big, booming bass underneath the seat, so I have to assume it's two identical 2.1s (where the .1 is for a subwoofer). So one figures that what NEC really wanted was a subwoofer, but they couldn't get one with existing technology (though it'll work fine over Dolby Pro Logic II); however, being that the soundtrack already encoded "surround" the exact same way Dolby Surround does, might as well cash in on the compatibility and market it as a game that will put your system to use the way it's intended to be used.
The thing is, tons of other games have used stereo for a "surround" effect. I'd be surprised if it weren't as old as stereo output in games itself. The reason why it was heavily marketed for Super Darius is because NEC wanted to give the impression that it could offer what the arcade cabinet did. This port was released alongside a different port, Darius Plus on HuCard (cartridge), which has an imitation of the soundtrack due to using its different sound hardware rather than Red Book CD recordings of the original hardware, but all of the sounds are essentially similar, including a "surround" effect that, if everything I have here is correct, is identical. That is to say that this game could be pushed through the same receiver and work as well as the other game.
What's probably more special is Dolby Digital, which requires higher fidelity cables than your standard audio jacks and represents "real" or "good" surround today (the superior Dolby Atmos notwithstanding). The first game with Dolby Digital support was Wing Commander IV on DVD for Windows. Over the course of the sixth generation, Dolby Digital, DTS, and surround sound more broadly would come to be supported in hundreds of games, especially for the Xbox.
Incidentally, the reason I made this post is because I felt inclined to look into what the emulation situation is for these technologies. While most emulators don't have decoders, the hardware you're running them on can be run through any kind of receiver (such as a Logitech Z906 PC surround system, which has one built in) and it'll work fine.