They probably mean sapient. We consider most animals sentient but only humans sapient.
That said, rights don't happen through scientific discovery but through reform or revolution. There are strong economic incentives to keep animal rights minimal and there is only very weak political will to change that. It also doesn't help that in the US there are strong religious doctrines that oppose the notion of animals deserving rights at all.
It's worth mentioning that there are indeed jurisdictions (not sure about the US) where animal cruelty laws are actually framed as animal rights (usually with some limitations, e.g. only considering vertebrates) rather than as protection of private property but even they usually don't include a right to life or bodily integrity and are flexible about what constitutes cruelty and what can be done to reduce it.
Aren't all animals sentient? And what is the difference between "truly sentient" and "sentient"?