It is the government's job to prove you remember it. They have the burden of persuading the court that you don't remember it. This is spelled out explicitly in the Supplemental Order issued by the judge, PDF freely available at:

https://ia601303.us.archive.org/0/items/gov.uscourts.paed.50...

What is at stake is a burden of evidence production. The difference between production and persuasion, in a court room, is a distinction both of quantity and quality. You "produce" single pieces of evidence, whether weak or strong -- then the totality of all of the pieces of evidence, evaluated together according to their strengths, "persuades" the court. "Produce" is quality-agnostic and refers to single pieces of evidence; "persuade" or "prove" refers to all of the evidence on both sides and weighs them by their quality.

And what's really at stake is that your lawyer, at a contempt hearing, cannot merely claim, "Well, your honor, the prosecution has not proven that my client is definitely able to fulfill the order, because they have not disproven every possible reason why my client could not fulfill the order." The reason that this is not enough is that it is asking the government to prove a negative, "you cannot possibly have any excuse."

Instead the procedures that courts use to decide these cases require the defense to identify specific reasons why the defendant cannot fulfill the earlier order, and then the burden falls on the prosecution to prove that those specific reasons are BS. And for this purpose they need to produce some evidence -- it doesn't need to be strong, persuasive evidence -- that the defendant cannot fulfill the order for that reason. Because it does not have to be strong or persuasive, a statement of testimony is taken at face-value as sufficient.

The only attack the government can make at the evidence-production level is "even if this evidence were 100% persuasive the defendant would still be able to decrypt the drives", so the evidence has to have some clear logical relation to the excuse. But obviously if your sworn testimony that you cannot decrypt the hard drives were 100% persuasive, you would not be able to.

It's still the government's job to prove you remember it, but you need to produce some sort of pertinent evidence (no matter how flimsy) that you don't, so that the court even evaluates this defense in the first place.