He eventually complied late at night on July 11, later clarifying that he had misread the letter.
Out for the count?
No, Mr Au returned with another post on July 15 questioning the contempt of court law and arguing that by not being able to comment on the judgment, his freedom of speech was curtailed.
Yesterday, the AGC responded and said Mr Au got it wrong. In a statement, a spokesman said the law of contempt exists to protect public confidence in the administration of justice.
The spokesman added: "A judge can be criticised, even fiercely criticised for getting the law or facts wrong, for getting the decision wrong or for imposing the wrong sentence. This is regularly done by lawyers, academics and lay persons.
"Such criticism is not contempt. There is no curtailment of free speech that would prevent such criticism.
"It is contempt, however, to say that the court was biased if there is no objective rational basis to do so, as Alex Au did."
The spokesman said Mr Au's June 18 post went beyond merely criticising the judgment.
The statement continued: "He deliberately misrepresented the facts, and then accused the court of being biased, on the basis of his false facts. This is very wrong.
"To make his points sound valid, Alex Au decided to mislead. Thus AGC asked Mr Au to remove his remarks setting out the false facts, and apologise for making the contemptuous remarks of the judiciary.
"It is misleading of Mr Au to now allege that our laws on contempt prevent debate and curtail free speech without acknowledging what he has done."
A knockout blow? Hardly, said observers.
Not even down for the count.
Political watchers said the exchange was a marked contrast from the iron-handed approach of the past.
Individuals such as Singapore Democratic Party leader Chee Soon Juan, as well as three men who wore T-shirts depicting a kangaroo in judge's robes (implying a kangaroo court characterised by unfair legal proceedings) have previously been sentenced to jail for contempt of court in 2006 and 2008 respectively.
Socio-political blogger and former Nominated Minister of Parliament Siew Kum Hong said that "it is very encouraging that the AGC has chosen to engage in a public debate on this important issue of public interest, instead of taking legal action".
"This was consistent with their earlier action of not prosecuting Alex for contempt of court if he published the apology letter."
He added that "this is all part and parcel of the so-called 'new normal', and everyone who participates (including office-holders, politicians and the public) must be ready and willing for robust debates."
Dr Tan Ern Ser, faculty associate at the Institute of Policy Studies and sociologist at the National University of Singapore, believes that "the Government would rather not resort to legal action", citing the new political climate that has evolved since last year's General Election as the main reason.
In fact, such a pattern of exchange would serve as a reminder for citizens of the "acceptable procedures for raising questions of reasonable doubts, without committing 'contempt of court'," he added.
He said that such doubts, if not shared openly in the social media, can remain unaddressed in public opinion, which may lead to more misunderstandings.