So Manafort is in solitary because of witness tampering, yet Mueller tampered with the secrecy of the grand jury and all is good.
Got it.
Nothing in the article itself suggests a violation of grand jury secrecy. The gist of the article is that, in exchange for a vague reassurance that they - the AP people - “understood†Manafort’s business dealings, the AP reporters appear to have given the FBI some useful leads. As the article states, the flow mainly went the other way (ie from AP to FBI, not the other way around).
Most of which is irrelevant to why Manafort is sitting in prison: he attempted to tamper with witnesses. That is a very serious allegation because the reliability of witnesses is one of the fundamental cornerstones of justice, without which a trial cannot be fair. Grand jury secrecy not so much.