

Yes, that’s why qBittorrent’s “Download first and last pieces first” option exists. You’ll be able to stream that .mp4 with its moov atom at the end of the file as long as you download that last piece during the beginning of the download. In some ways that makes qBittorrent a better streamer for .mp4 files vs other methods.
Maybe the other commenter is referring to some other media file type that can’t be streamed.
EDIT - Haven’t checked but am guessing torrentio also downloads first/last pieces first otherwise it’d be a terrible torrent streamer.
This doesn’t answer your main question but I suspect their thinking is that VPN use is already very common in torrent communities. So common that it’s not really a big deal for users to browse the site via VPN if necessary. It’s not really that the staff there is going to do age verification, realistically if necessary they’d likely just block access from whatever states/jurisdictions are giving them issues with age verification.
That aside not too sure if what you’re looking for currently exists. There are a few general torrent sites that also operate Tor / I2P domains, and also the general torrent indexer Postman exists on I2P. The amount of users in Tor / I2P networks is way lower vs the clearnet torrent sites so a hyper-focused torrent site in Tor / I2P doesn’t seem viable but I could be wrong.