I believe that Dunn was not privy to the details and wrongly assumed that the $17B offer did not include an assumption of debt.  If the offer was net to shareholders (meaning GDF Suez also assumes all current liabilities)  the value of the offer was $16+ share.  This seems more believable since TLM has been trading between $12. 12 - $13.25 over the past 90 days.  Common sense would dictate that any takeover offer would not be below any weighted average share value just prior to the offer.  Usually a premium to that share price. 
The article states "French electric and energy utility GDF Suez SA tried to buy Talisman for $17-billion including debt"   Now did they mean $17B + Debt or $11B + Debt.  Time will tell