May peace be upon you
As we note from the next ayaa.
"The fornicator does not marry except a [female] fornicator or polytheist, and none marries her except a fornicator or a polytheist, and that has been made unlawful to the believers." (Quran 24:3) SAHIH INTERNATIONAL
So here the fornicator is allowed to marry a polytheist, which is forbidden to a muslim.
"And do not marry polytheistic women until they believe..." (Quran 2:221) SAHIH INTERNATIONAL
As my understanding, this aims to the non-muslims under the leadership of a muslim leader (so they have to follow its rules), this also raises another point. The fornicator is forbidden to marry the believers (Quran 24:3), so they are not even identified as believers here by God! This makes the sin of fornicating extremely great as God doesn't even consider them believers. At the end God is forgiving and merciful.
"Except for those who repent thereafter and reform, for indeed, Allah is Forgiving and Merciful." (Quran 24:5)
Hope this answers the question and also an insight. This is what I am convinced of and if I made any mistakes, please let me know.
Thank you.