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CD-i players as such predate the PhotoCD, but the PhotoCD standard prescribes that every PhotoCD must contain a CD-i application to play the disc. In those days you could buy a suitable application from Philips.

I suspect that many "modern" PhotoCD authoring tools no longer include the CD-i application on the disc; this would make the disc indeed unplayable on a CD-i player.

You can use IsoBuster to check if there is a CDI folder on the disc with something that looks like an application; if not then it surely won't play on any CD-i player I know of.