The Burdick case had to do with an individual who had not yet been convicted of anything being offered a pardon in exchange for testimony that could have otherwise incriminated him. The Supreme Court ruled that in that specific scenario someone accepting a pardon could be seen as admitting guilt, so the pardon couldn't be forced on Burdick to strip away his fifth amendment right and compel his testimony.
A pardon does not automatically require a confession (though it could be part of the terms), however blanket pardons remove the ability to plead the 5th on the topic if subpoenaed as a witness. The 5th precludes being forced to testify against yourself, but if the government has declared it not a crime then you can’t testify against yourself even if you wanted to.
From what I recall, non-global pardons still maintained some level of 5th Amendment protections. Ie you were pardoned for A, but talking about A might reveal you did B, so you could still potentially testify against yourself.
Iirc, non-global pardons usually come with a stipulation like “must testify that or about…” because of this.
These applications of the pardon power have been controversial, but never successfully challenged.