Sweden finished a point below second-placed Japan in Group F, but progressed as one of the higher-ranked third-placed teams.
But Elanga, believing they needed a win, was pushing his teammates to find a second goal and even pulled up with cramp, earning himself some harsh words from his teammate Alexander Isak.
"He got a little scolding from me there. He was a little frustrated at the end of the match, and you can understand that now," Isak told reporters after Thursday's match.
Elanga said his teammates were trying to shout at him, but he did not know that they only needed one point to qualify until the match was over.
"I just shouted, come on, we can go for more... I got cramp at the end but didn't want to stop running," he said.
Elanga was not the only player unaware that they had qualified.
"I was just as clueless," said Sweden goalkeeper Jacob Widell Zetterstrom, who made his World Cup debut against Japan.
Sweden coach Graham Potter could not help but smile about Elanga's confusion.
"It couldn't have been any clearer for him. He's obviously thinking about something else, bless him. I love him at the moment, but dear me," he told reporters.