I got the impression that it was a matrix effect, but I don't have a Dolby matrix decoder to test with.

Thanks lti, I was not aware of the existence of those matrix decoder. I searched about them and I think you are right, it's how they must have done it. Future Crew made it clear they used Dolby Surround and you need a stereo sound card, so I assumed they where using the Dolby Stereo and Dolby Surround (matrix) 4:2:4. Apparently it used to be called "Dolby SVA matrix" and output to 4 channels : left, right, center and surround (rear) using unobtrusively the stereo standard. The Center and Surround would be encoded with a phase difference in between the stereo channel. Looking at the samples it seem they did just that. A selection of "Surround" instrument are identified.

Skaven prepared many sample instruments that range from percussive instruments to sound effects. He denoted them with $ sign. I loaded "SWOOSH"and " SWOOSH$" as an stereo file to confirm they are inverted.

Purple Motion seem to have only inverted one sample #30 named "surround" that is an inversion of unnamed sample #29. The result is a similar channel inversion. It's the swish sound you often hear in between parts, for instance when you leave the pixel tunnel just before the moiré circles.

Here is a render of both music (2nd_skav.s3m+zoom on left and 2nd_pm.s3m+zoom at right) with only the surround instruments. Skaven implementation seem to be more ambient, although most of the time he would not use it's surround counterpart. I think the place where his surround effect is most important is at the "Swoosh" sound when you have the hyperspace blast shock wave fade to white. Even so, I put that sample in solo for the red zoom section rendering and it still look far less obvious than Purple Motion who seem to have a more perceivable approach using visual tools.

This mean that a Dolby Pro Logic decoder that support analog matrix encoded signal could render this music properly in surround. Or maybe there is a software that could help prepare an upmix for modern digital receiver using an recording or a rendering of the original audio ?

You can listen to the surround sound without a Dolby decoder, but not in real-time, by recording the demo to an AVI file with Dosbox, and then processing the wav file with Foobar2000 to upmix the audio to surround sound.

You will need to install a few Foobar2000 components:
Upmix the 2 channel audio to 5.1 Hydrogenaudio - FreeSurround
Optionally:
Get rid of the empty LFE channel Foobar2000: Matrix Mixer - SkipyWiki
Virtualize the multichannel audio back down to 2 channel for Headphones foobar2000: Components Repository - Dolby Headphone Wrapper

First, configure Dosbox to record the video to your hard drive. Make sure to enable Gravis at 44.1 Khz and the mixer at 44.1 Khz in the Dosbox config.

Run the demo and remember to end the recording.

Then, demux the wav file from the video file. Demux the video to another new file as well.

Configure the DSP filters in Foobar2000 in the order listed above, and save the wav file to a new name. Then multiplex the new wav file with the 2nd video file to a new video file.

EDIT: Forgot to mention, you can listen to the surround effects in real-time if your sound card has CMSS2, SBX Pro Studio, etc. But due to the limitations of the matrix encoding, there will still be cross-talk. The surround audio will never sound as good as discrete multichannel audio.

Using Stretch tutorial I was able to produce a 5.1 FLAC file that I could play on my living room's HTPC running Kodi on Linux. The surround effect is very exciting and I was amazed to hear the front/rear separation of some effects ! The big spaceship after intro scrolling clearly come from the rear and fade into front. I feel now to make an HEVC 1080p DTS 5.1 video is the only sensible thing to do ! 🤩

I used OpenMPT to render the two .s3m music as it was more convenient now, but I assume the audio from a DOSBox movie would work as well. FreeSurround have an option called "Redirect bass to LFE (.1) channel" that is off by default. On this screenshot you see Purple Motion on top with the option on and on bottom is Skaven with the option off.
6 channels: front left, front right, front center, LFE, back/surround left, back/surround right

On my receiver both version and the subwoofer sound the same to me. I understand next step would be to not even bring this channel to the export using MatrixMixer.

Thank you Stretch and lti, with your help I am now able to enjoy this soundtrack in surround on top of all I learned about dolby doing it.