Followup question: Wasn't Ethiopia occupied by Italy back then? How did he implement this law without controlling country? Wouldn't be slavery illegal in Italian law?

I will admit, I had never examined the topic from this specific angle before.

After a little bit of digging through African American newspaper online archives, I am not finding any headlines specifically about the August 1942 announcement abolishing slavery. Unhappily, the articles are behind paywalls, so if there is any incidental mention of slavery's abolition, I can't see it. The tone of most articles that mention Haile Selassie or Abyssinia between 1942 and 1943 tend to focus on reconstruction efforts, requests for aid, and Ethiopian diplomatic and military efforts in the ongoing war against Italy and Germany.

However, in between 1922 and 1935 there was enormous focus on the issue of slavery in Ethiopia/Abyssinia, in both the mainstream press and the black press. The issue was first raised in connection to Ethiopia's admission to the League of Nations, and the requirement of ending slavery to gain admission. In 1935, the issue saw new life as tensions between Ethiopia and Italy saw Italian propaganda play up the issue of slavery to present Ethiopian society as barbaric and in need of a "civilizing mission".

Broadly, the American press was divided in whether they presented a vigorous attempt to end slavery or an entrenched problem

Specifically in the black press in 1935, we do see articles arguing Ethiopia was more committed to ending slavery than Italy was in her colonies and playing up instances where the Italian army used forced labor in their advance into Ethiopia.