Doing some simple research, the first two things advertised on their website's store were Veggie Tale adaptations of "O Holy Night" and "The Little Drummer Boy" so...
I think your friends are wrong, respectfully.
They are definitely Christian-themed. Yes, they have a number of episodes that include only Old Testamant/Torah stories, and also some "secular-esque" shows (the Larry Boy bits, specifically) that teach morality without being strictly bible-based. But they have also done several episodes about Easter and "The True Meaning of Christmas." I am pretty sure I remember an interview in which Mike Nawrocki says that they did this deliberately to be inclusive of Jewish and Muslim families, and families that are "spiritual but not religious." Still, if you are a parent who wants to make sure your kids aren't exposed to overt Christian themes, you'd have to put in the effort to pre-view the episodes before you let your kids watch them.
You might want to tell your Jewish friends this, actually -- if they're parking the kids in front of VT without knowing that even some of the Hebrew Bible stories have quotes from the NT at the end, they might want to re-think their blanket approval of all things Veggie. Depending how strict they are. I know another Jewish Mom who lets her kids watch select Veggie Tales, but she also uses it to talk to them about the differences between Christianity and Judaism.