Certain zip codes are not listed in the geotargets list, because you cannot target them.
And yes, the geotargets list only contains geographical areas that you can target in Google Ads.
If you intend to target an unlisted zip code, which you can target in Google Ads web interface, do realize that the geotargets list is only updated a few times per year. You might want to ask the API team to make sure that zip code is included in the next iteration.
If you intend to target an unlisted zip code, which also cannot be targeted in Google Ads web interface, you may want to contact your Google representative and explain why you need that targeting.
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