That's an interesing theory.

I had always interpreted this line differently than you.

I had thought that Sollozo was saying that 10 years ago the Don would have taken the deal that Sollozo was offering.

"Gotten to him" in my mind meant that Sollozo was saying "I could have persuaded him to take this good deal 10 years ago". But now the Don is too old and cautious and risk-averse to something as brash as what Sollozo is offering to the Corleone Family. The "slipping" line meant that the Don was no longer aggressive enough to try new "business" opportunities and therefore Sollozo could not "get to him."

But I think your theory is probably better.