I don’t understand how this is a “threat” to the 5th Amendment. This person was not compelled to talk to or give a statement. One has the inherent right to not speak to law enforcement.
You're not understanding how this works. Law enforcement will lie/pressure you to talk. They use psychology techniques. They will hold you against your will. Implied threats. They'll say "witnesses claim they saw you assault the victim" when that isn't true. They will claim nonexistent video evidence. Weak minded people will crack under duress such as this.