A young platform with an inconsistent media pipeline
Threads launched with Instagram's infrastructure underneath it but its own posting rules, and the media handling still shifts from release to release. Video length limits, supported aspect ratios and the exact renditions served have all changed more than once. That is why a Threads downloader has to read whatever the post exposes today rather than assume a fixed quality ladder.
In practice most Threads video comes back as a 1080p-class MP4 with a bitrate similar to an Instagram feed post, since the same encoding pipeline handles both.