Is it correct that possession of a firearm in Spain is only punishable by a fine but not imprisonment?
It depends...
Yes: "If they do not constitute crimes" as per the OP's linked article, otherwise:
No according to Article 564 of Spain's Criminal Code pdf:
- Possessing regulated firearms while lacking the necessary licences or permits shall be punished:
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- With a sentence of imprisonment from one to two years for handguns;
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- With a sentence of imprisonment of six months to one year for long firearms;
- The felonies defined in the preceding Section shall be punished, respectively, by prison sentences of two to three
years and from one to two years, when any of the following circumstances concurs:
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- When the weapons lack factory marks or serial numbers, or have these altered or obliterated;
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- Which have been unlawfully imported into Spain;
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- Which have been transformed, modifying their original characteristics.