That is true, but consider that using leather for armor was rare, as it's ineffective. So if ever we see a movie with leather armor, there is a high probability that it's not realistic.
The most common types of armor have been linen or wool, such as linothorax or gambeson, mail armor, that the romans used extensively, and plate armor, such as lorica segmentata. Though this last one was only used for around 300 years.
People don't need to have a degree in Roman history to see those armors look like complete trash.
But the thing is, there reason not to use accurate armor. Leather armor or mail armor will have similar costs. For a movie, it's not even needed to be riveted mail, making it even cheaper.
And if they just want something stylish, they could have just used lorica musculata. This was used mostly for parades and such, but it's something a high ranking officer would use. And it looks much better than brown/black leather.
This production of Gladiator 2, just looks too amateurish. It's plainly obvious that the director, the producers and the costume department don't give a flying fuck about what they are doing.