Tyrell died on the night of May 24 after two separate blows to his abdomen that caused internal bleeding and the leaking of his stomach contents, known as peritonitis.
Strbak pleaded guilty to her son's manslaughter in November but denied she delivered the fatal blows.
Defence barrister Greg McGuire had argued the circumstantial case against Strbak's then-partner Matthew Scown was stronger than against the boy's mother.
But Justice Peter Applegarth on Monday found there was a "no compelling evidence that Scown inflicted the fatal injuries".
Justice Applegarth found Strbak was not only responsible for the serious damage that killed Tyrell but also for many of the non-fatal injuries inflicted on him in the days before his death.
Tyrell had 53 bruises and 17 abrasions from head to legs when he died.
Strbak could be seen rocking back and forth in the dock and shaking her head as Justice Applegarth handed down his findings.
She also appeared exasperated as she spoke to her lawyer after the court closed.
The 34-year-old will now be sentenced in the Brisbane Supreme Court on December 18.
Scown, also 34, was sentenced to four years' jail after pleading guilty to manslaughter but walked free from court in October, having already spent two years and eight months in custody.
He was sentenced on the grounds he was not responsible for Tyrell's injuries but failed in his duty of care by not seeking medical help for the boy.
