Does 少数民族 exclusively refer to the 55 Chinese ethnic minorities?
From your link, 少数民族 = 多民族国家中除主体民族以外的民族, so that term applies to other countries as well. However, 中国少数民族 designates exclusively Chinese ethnic minorities. Keep in mind that there are 中国未识别民族（Unrecognized ethnic groups in China）, i.e. ethnic groups too hard to identify due to strong similarity with other ones or ethnic groups influenced too much by Hans.