The Prophet (ﷺ) told Muslims not to plunder.
It's true that the tribe of Ghifar were previously known raiders however in the end they embraced Islam and were pardoned by Allah (ﷻ). After repentance, people are not held to their 'old' reputation nor can we make assumptions about them based on their old ways. They start with a clean new reputation.
With Abu Dharr (RA) after conversion, he is mostly described in biography books as being involved with Da'wah, speaking truth, being at the Prophet's service and close company for seeking knowledge and his asceticism (withdrawing from the world).
This is the first I'm hearing of "continued raids". And I'm not quite sure of the accuracy of the account because within that same section it says Abu Dharr easily found the Prophet ("without difficulty he found his way to the Prophet's house") whereas various hadith show otherwise, that it took him days to finally meet the Prophet (ﷺ) and, per one account in Sahih Muslim, even his inquiring about the whereabouts of the Prophet (ﷺ) ended in the Quraish beating him.
So I would try to find some further collaborating scholarly materials to first verify the claim from your book is accurate. (Note: the only example given by that source is Al Quraysh who were hostile to begin with; no other examples were given).