I'll start with my disclaimer again. I am not an investor, but...
One presumes that they'd have to repatriate all the money in order to buy back shares. I have no idea what the going tax rate for such an undertaking is, but I suppose from his statement it would levy a greater than $20bn charge, requiring borrowing.
More likely is Icahn is hoping Apple would use their cash as collateral to borrow a truckload of money at a rate of interest which would incur a far lower charge than repatriation would, keeping their "war chest" more or less intact, just like they did recently.
Still, it seems likely - to my naive, non-investor eyes at least - what Icahn's after… He "invests" a couple of bill in Apple shares and manages to convince Tim to buy them back. Since buybacks often (temporarily) inflate the share value, Icahn profits.
This is repeated until it becomes clear that Tim is not stupid enough to buy back such a preposterous and pointless number of overvalued shares ($150bn what the serious fuck?) when in fact their track record shows that they actually tend to only buy when the shares are nicely undervalued by the market... At which point, investors turn bearish as/and price declines. Whereupon, -just by sheer coincidence it happens to turn out that- Icahn has in fact shorted Apple for a couple of billion and profited again.