1000s of developers use Idea's IntelliJ, Webstorm and GoLand. Only the community edition of IntelliJ is open source.
Software doesn't need to be open source for it to be adopted, if they have the right security practices and I'm sure for enterprise contracts they will have the right level of information available under NDA when GA.
Enterprise contracts also include clauses spelling out financial consequences of major screwups. If Jetbrains screws up and a customer's passwords go everywhere, I'd bet the contract makes Jetbrains at least a bit liable.
