Aha; well if you read the book more closely, you'll find that Thompson merely states that there's a rumour floating around that Muskie is addicted to Ibogaine.

Further, at some later point in the same book, he addresses it by pointing out that a) he was only reporting a rumour and that b) he himself started the rumour.

From that, it's obviously highly likely that the Ibogaine story was just a product of Thompson's imagination and sense of humour.