It's probably harder to tie devices in a single household together with IPv6 than with IPv4, or so they think.
They don't make (much) money on their devices, they make money on selling data collected and aggregated by their devices. Their "privacy" policy [0] is a hoot to read. "Whatever we can do, we will. If the laws of the country we collected the data in would interfere with that, we'll move the data and then do what we want." I mean, they grant themselves permission to nmap your network for other devices! Emphasis mine.
> We may receive information about the browsers and devices you use to access the Internet, including our services, such as device types and models, unique identifiers including advertising identifiers (e.g., for Roku Devices, the Advertising Identifier associated with that device), MAC address, IP address, operating system type and version, browser type and language, Wi-Fi network name and connection data, __and information about other devices connected to the same network__.
[0]: https://docs.roku.com/published/userprivacypolicy/en/us