Rambam also uses the term tikkun olam to improve or perfect society — here’s Chabad’s translation of Mishneh Torah Rotzeach uShmirat Nefesh – Chapter Two: “When a Jewish king desires to slay any of these murderers and the like – who are not liable for execution by the court – by virtue of his regal authority, in order to perfect society, he has the license.” This is consistent with a general use of the term to mean improve or perfect society.
The problem is not using the term tikkun olam to refer to improving society. The problem is what kind of improvements they have in mind (and the Reform idea that general efforts to reform society are the most important aspect of Judaism).
Sure, helping oppressed orphans and widows and crime victims, stopping human trafficking, giving employment training to unemployed people, etc. are are good things to do under the Torah and they improve society, so we could use the term tikkun olam for them as an informal matter. But of course we should oppose people using that term for promoting causes that are against the Torah, as some non-Orthodox do.