<i>"What possible justification can they have for not mentioning GH or the SES?"</i>
As I pointed out in a post to MikeS further up this thread, the NIO press release <i>does</i> mention SES:
<i>"A team of underwater archaeologists from National institute of Oceanography NIO have successfully `unearthed’ evidence of submerged structures off Mahabalipuram and established first-ever proof of the popular belief that the Shore temple of Mahabalipuram is the remnant of series of total seven of such temples built that have been submerged in succession. <b>The discovery was made during a joint underwater exploration with Scientific Exploration Society, U.K.</b>"</i>
As I also pointed out to MikeS:
Agreed there's no mention of GH's instrumental role in bringing the two parties together, and the role of the SES tends to be downplayed, but contrary to what you suggest above they do get a mention.