It runs in my mind that the claim was made during discussion on the forum ... I've carried out a search but haven't come across it again.
I've got to admit it certainly doesn't sound kosher and Davel is correct to request a cite. Maybe someone can come up with one ... or maybe will even remember the discussion. So far it seems it is just you and I who seem to recall it.
Try looking in "Toward a Prosecutor for the European Union Volume 1: A Comparative Analysis".
Section 18 pages 540 through 586.
In the full knowledge you will have to buy it at the paltry sum of 150 Euros or go to your reference library this is what it says:
"The Portuguese procedural system assigns the arguido the right to remain silent during the procedure from the moment he/she is formally designated arguido/arguida.
The fact that the arguido decides to remain silent cannot be detrimental to himself. Moreover the CCP does not threaten with a sanction of anykind the arguido that having decided to talk instead of remaining silent makes false declarations.
This does not confer a right to lie merely a right not to be punished for lying.
As a consequence of the right to remain silent the arguido has no duty of any kind to cooperate with investigative authorities for the purpose of determining the truth.
It should also be noted that as a refraction of the privilege against self incrimination the arguido cannot under circumstances make a declaration under oath.(CCP Article14(3))".
There is also a section relating to witnesses and a very informative bit re suspect definition and limiting time for arguido status. Not to mention sections for all member states of the EU which are also informative.
I'll leave you to buy the book or visit the library.
If you think the book is unreliable refer back to the EU they commissioned it.
p.s you could also try wiki
The rights of an arguido
"If a person becomes an arguido, they automatically gain certain rights that a witness or suspect would not have.[7] An arguido has the right to be accompanied by a lawyer when questioned.[5] The investigating police may ask the arguido more direct accusatory questions (the answers to which would not be admissible in court if possibly self-incriminatory and asked of a non-arguido) but the arguido must be presented with whatever evidence is held against them,[7] and unlike a witness has the right to remain silent,[8] not to answer any question that may incriminate the person, and does not face legal action for lying.[9]"https://ipfs.io/ipfs/QmXoypizjW3WknFiJnKLwHCnL72vedxjQkDDP1mXWo6uco/wiki/Arguido.htmlPick the bones out of that!