That was true when it was at Westside, and they used the same clay as Roland Garros. It's at River Oaks now, is there any info on the tournament website about the surface?
There is no info on the tournament website, but as I said the company that did the courts says it is their American Red Clay product which also bears the HAR TRU label as seen above.
For more confirmation of the surface there is this article which also describes the courts at River Oaks as American Red Clay, installed in 2005 (note that this article is about River Oaks winning the rights to the tournament and was written after the final one at Westside):
and then there are of course all of the photos floating around of the court in which you can see it really doesn't look like normal red clay, but rather closer to a maroon or brown HAR TRU. for example: