Maybe so, but the place for that information is in church, not at a public graduation. In a church, people come because they want to hear the word of God. At a graduation, people come to see their friends and relatives celebrate an accomplishment. It is appropriate to thank God in this setting, but entirely inappropriate to condemn those who are other religions. The message may not be wrong, but the venue certainly was. Imagine if you were a religious Jewish family who had come to celebrate your child’s graduation. How would you feel if the valedictorian told you and your family that you would all burn in hell?
I honestly wouldn't care. I might even laugh. People can say whatever they want, meanwhile, I can choose to listen or not or even discount what they say. Maybe I'm just not offended that easily.
Being a minority in America and growing up hearing sly and racist comments from both white and black people may have something to do with my being inured to this, however. The Gospel is just not offensive to me no matter where or how it is presented. It wasn't offensive to me back in the days when I was not a believer, either--I just ignored it, and quite a few people still do.