I think that’s quite right. Many things in our culture have religious roots or religious connections, yet also have a secular dimension; there is no Establishment Clause problem with the government using them in a secular way. That the government may not engage in rituals involving the body of Christ doesn’t make it unconstitutional to have a city called Corpus Christi (or Providence). Holidays that have a secular dimension as well as a religious one — such as modern Thanksgiving and Christmas — may be made into days off.
Religious songs can be sung by public school choirs, and religious paintings can hang in government-run museums, because of the cultural, artistic, and historical significance of those works. Even Sunday closing laws have been upheld on the grounds that those laws have the secular function of creating a uniform day off that is convenient to the maximum number of employees and small businesses.
To be sure, one can argue that yoga classes make yoga into something familiar and appealing, and may thus make some “students more susceptible to Hindu religious teachings at some unspecified time in the future” (to quote the court’s characterization of the plaintiff’s argument). But that can’t be enough to make it unconstitutional, just as the possibility that someone seeing a religious painting in a government museum may be moved to become more religious isn’t enough to make it unconstitutional to display great religious paintings for their secular artistic and historical value.
Obviously, one can imagine overtly religious yoga classes, which the court notes may well be unconstitutional, as well as classes that come closer to the line. But this class was apparently quite thoroughly focused on the secular physical and mental aspects of yoga, and that made it clearly permissible.
Thanks to How Appealing for the pointer.