The issue is spelled out in Ibrahim at para. 164/170 (characterizing the Government's argument/characterizing the applicant's argument).
-Adverse use of exculpatory statements made under compulsion is violation of right against self-incrimination (Saunders)
-Adverse use of false "freely-made" statement is not violation of right against self-incrimination (Allen)
The issue would be whether there was "compulsion", which in my view is not a well-defined concept under ECHR caselaw. In my view there was compulsion and compulsion is easily found; however, I think that they would have had a shot at proving lack of compulsion.
But then that argument could be used in the current "simple" calunnia case.
I need to further digest Allen v the UK, but she was actually acquitted for manslaughter and the ECHR case I am looking at was about her being compensated.