In this sense, free market maximalism is a materialistic religion of its own.
Still, this is not the framing that people calling transhumanism a religion use. The arguments are usually trying to show parallels between transhumanist beliefs/theories and theistic religious beliefs (almost always Christianity) - specifically to say, "look, it sounds like Christianity if you substitute AI for God and Singularity for Rapture, therefore this is just rebranded Christianity, not Science", therefore implying the transhumanists are wrong. The argument is, literally, "this pattern-matches to religion therefore must be just as wrong", and I'm saying it's a dumb argument.
A good example of atheistic, even materialistic, religion was the official doctrine of the USSR, "marxist dialectical materialism". This had all of the hallmarks of a religion, complete with religious texts, priests, condemnation of non-believers, and so on - but caked in fully materialistic and dogmatically atheist concepts.