Question: Is there a distinction between 空港 (kōnggǎng) and 机场 (jīchǎng) which both mean airport?
空港 is Japanese; 机场 is Chinese.
Japanese learned Chinese characters from China in ancient times. They called those Chinese characters 'Kanji'. Most Chinese terms retained their original Chinese meaning in Japanese. But the Japanese also coined some unique terms with Kanji that are not being used in Chinese.
The 空港 sign you saw in China was reverse imported from Japanese.
和製漢語 由於漢語也是因為19世紀時急迫需要翻譯西文，而大量從日本輸入新詞，大部分進入漢語的日語借詞都是日語中的「和製漢語」。舉凡「電話」、「幹部」、「藝術」、「否定」、「肯定」、「假設」、「海拔」、「直接」、「警察」、「雜誌」、「防疫」、「法人」、「航空母艦」都是屬於和製漢語日語借詞。。 要注意的是，並不是所有日語中的「和製漢語」都在現代漢語中通用，如「介錯」、「怪我」、「油斷」這類和製漢語詞都沒有進入漢語。
空港(airport) was in the group of 'Japan coined Chinese' that are not in Chinese vocabulary - until now.
空港 (くうこう) is a standard Japanese word, 机场 is a standard Chinese word. According to Xiàndài Hànyǔ Cídiǎn 7th Ver, which is a authoritative Chinese dictionary in Mainland China, 空港 isn't included.
In other words, 空港 is an unstandard and not-often-used word in Chinese, so that you'd better not to use it during comminication and you should not use it in Chinese examination host by Mainland China.