New signing Gareth Dean made his debut from the substitute's bench but the Welsh international couldn't help the Scorpions as they suffered a heavy defeat.
Rochdale led 24-14 at the interval and went onto boss the second-half as Seibold's hopes of a play-off finish took a major blow.
Poor defence allowed Dean Gorton to break the deadlock for Rochdale, though Lee Williams' equaliser soon after restored parity.
Lloyd White's penalty put the Scorps 8-6 to the good, but Rochdale never looked back after John Crookson's 25th-minute try and went onto dominate proceedings.
Damien Reid scored soon after with the first of a fine triple before Danny Smith scored.
Jamie Murphy's try on the stroke of half-time and White's brilliant conversion reduced the deficit to ten - but the Hornets totally dominated the second-half with Dayne Donoghue and Dale Bloomfield scoring and Reid running over two tries.
The result leaves South Wales in seventh place - clinging onto a play-off spot by a single point with four league games to go. Rochdale sit 5th and should play their part in the end-of-season lottery.
The Scorpions travel to the Whitebank Stadium next weekend to face Oldham RLFC before entertaining Doncaster RLFC at the Gnoll. The season ends with tough games against two established sides – York and Hunslet. Full Time - Rochdale Hornets 46-14 South Wales Scorpions
Attendance - 495
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