Yes, though a major annoyance is that it's only the official SideFX plugins that are free, not Houdini Engine itself (the API). So you can't write your own specialized tools.

I ran into this recently when we needed an interactive rendering technique that Houdini doesn't support. The Houdini Engine API would have been perfect, but we use Indie and it doesn't support anything but the official plugins.

Instead, I'm writing HDK nodes in C++ that accomplish the same thing by copying to shared memory in a subprocess running Optix 7. It works, but it's ugly as hell. Oh well.