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You're welcome 👍
The film being 15 years out of date (and probably not stored in the freezer in the meantime) definitely is a factor as well, bit hard to say but maybe they did indeed use the right chemicals (it's mostly a bit "dull" colours and severe colour shifts (typical for very old / badly stored film) from what I can see, but not the super high contrast one usually gets from developing slide film in C-41).
Not sure what happened (would be easier to tell if you'd post some of the photos) - but yea, probably they just developed your slide film in C-41.
Some people develop their film in the wrong chemicals deliberately, then its called "cross processing" / "x-pro" - and this usually gives very strong contrast and a shift in colours; often blues turn to cyan very much etc...
https://www.lomography.de/search/photos?order=popular&query=xpro