The legal argument that I believe Mach has is: If a person is interrogated by police about one crime, murder [and the interrogation is against ECHR because the person has no lawyer], but the person (falsely) makes a statement implicating himself in a different crime - of a different type, calunnia, and then the person is tried for that different crime - calunnia - is the trial fair according to ECHR?
I believe since the calunnia in this case relates to the murder, it is certainly covered by the ECHR case-law. The calunnia was induced by the police coercion in their seeking information about the murder. And the statement itself is the crime!
If the calunnia had been about some completely separate crime, let's say a bank robbery clearly not related to the murder - then there could be some question about whether that calunnia was the result of coercion. Hypothetically, suppose that the police in their interrogation had asked a question such as: "Tell us what you know about the crimes of Mr. X ... or else you will go to prison!" In response, the person being interrogated (with no lawyer) then falsely states that Mr. X had robbed a bank. That statement is the product of coercion. I believe that a trial and conviction for calunnia in the hypothetical would be unfair according to ECHR. But if in the interrogation there were no questions about Mr. X or about bank robbery from the police, and the person being interrogated arbitrarily volunteered that he had observed Mr. X robbing a bank, then there may be a valid question of calunnia. But since the subject of the interrogation - which is conducted without a lawyer for the subject - may not understand his rights against self-incrimination and the laws against calunnia, I would say that no inculpatory information provided by the subject can be used against him, including any statement that could be calunnia.