Steele is a former FBI contractor. I have read elsewhere that the OIG doesn't have any method to compel anyone who is not an outside witness to testify, but I imagine if a former employee is willing to answer questions Horowitz and his team can talk to them.
I agree with the supposition that Steele suddenly became interested in talking to Horowitz when Durham was appointed.
One of the articles above says he was refusing to talk to Durham (criminal investigation) and now we hear he is willing to talk to Horowitz (FBI process investigation).
Once he does he may say "I had an agreement ... and use it to prevent him getting questioned by Durham (short of a formal extradition request).
Your comment is the first line only, the rest is my response.