While he stopped short of suggesting a controversial try before halftime awarded to Dylan Edwards cost his side the game, the inconsistency of how the rule is applied left fans baffled on Thursday evening.
The bunker ruled however that Ilias hadn't been obstructed as there was no chance of being able to catch Edwards and make a tackle.
Speaking post-game, Demetriou said he had given up.
"Honestly, I've given up trying to figure out what that rule is," he told reporters.
"(Tago) stops in the line. There's no way Lachie can get off that."
Social media was ablaze after the incident, with the NRL refereeing group constantly in the headlines this season for controversial decision after controversial decision.
Despite the try to Edwards, South Sydney still had the chance to win the game, only for a late Penrith try to pick up the win for the men from the foot of the mountains as they closed out the minor premiership.
Demetriou targeted his side's defence as a key problem out of the contest - something South Sydney have often struggled with over the last 18 months, having conceded 50 points or more twice last year, and currently holding the second-worst defensive record of teams in the top eight this year.
"We shouldn't have conceded 26 points in a game when they're missing their two most creative players," he said.
"We needed to get in front on the scoreboard and put some pressure on them to make a play and we weren't able to do that up until the last few minutes.
"A few blokes missed their assignments in defence tonight and that's what cost us."
While the win for Penrith secures the minor premiership, the loss leaves South Sydney in fifth spot on the table, but with the potential to fall to eighth if the Broncos, Eels and Roosters all manage to pick up victories this weekend.