To be fair, it also doesn’t look too dissimilar to America’s Mark 48 torpedo that remains in service — this is in large part because it’s believed China stole elements of that torpedo design during their development of the Yu-6.
One defense journalist took to Twitter to offer a translation for the visible characters on the torpedo, which say “connect” and “disconnect,” in characters indicative of the People’s Liberation Army-Navy.
Based on the evidence available, it seems likely that the torpedo was launched from a PLA-N submarine elsewhere in the South China Sea, and likely within the confines of what other nations would consider international waters (despite China’s refusal to acknowledge that any water in the South China Sea falls under the international classification). The dummy torpedo then drifted at sea until it was stumbled upon by Tran Minh Thanh and dragged back to shore.
You can see more of the torpedo in the local news report below: