I know of no such exception that would apply to US Citizens. Even resident aliens, who are taxed the same as US Citizens, would be taxed on such sales.
The only possibility would be if there is a "tax treaty" provision with the specific country that you are referring to, that could offer such an exemption, but that would require a review of the specific tax treaty between the US & your "home country".
If you give me the name of the country involved, the date you acquired the property, the nature of the property (for example was it your primary residence or was it rental property), and the date you became a US Citizen, I can determine if a further review would be warranted, and if so provide you with an offer to perform a review of any tax treaty between the US and your home country for an additional fee.
The other option that you may wish to explore is to get the specific reference to the law or regulation from whomever provided you with the information that the sale wouldn't be taxed to a US Citizen in your circumstances. Often, these type of statements are made in a confusing manner and don't universally apply to everyone's specific circumstances.
One issue that often works to confuse things is that as a US Citizen, depending upon the country involved, you may be entitled to a credit against your US taxes for any tax paid to your home country on the sale of the property, thus possibly reducing or eliminating the US tax on the sale. Possibly, that could be what the embassy employee was referring to. It is hard to know without a specific reference.