WRONG. Grumman had already planned for that scenario nearly four years prior to the accident.
The Grumman-directed Apollo Mission Planning Task Force reported on studies of abort sequences for translunar coast situations and the LEM capability to support an abort if the SM had to be jettisoned. The LEM could be powered down in drifting flight except for five one-hour periods, and a three-man crew could be supported for 57 hours 30 minutes. It was assumed that all crewmen would be unsuited in the LEM or tunnel area and that the LEM cabin air, circulated by cabin fans, would provide adequate environment.
The Apollo Spacecraft: A Chronology, 1966 May 9
In addition, Apollo 9 was a test of the lunar module in low Earth orbit. It included starting and re-starting the descent stage engine with the command/service module attached, just as was done in Apollo 13.