For the record, according to the records of General Motors, as of April 21, 2009 there were still 57 workers at the Janesville plant building its last medium-duty truck and rolling it off the assembly line. About 20 workers continued "closing up" until June 2009. Obama, not Bush, was in office.
In February 2008 at a campaign stop in Janesville, Obama says, ”I believe that if our government is there to support you, and give you the assistance you need to re-tool and make this transition, that this plant will be here for another hundred years.”
The plant was slated to be closed under Bush, and indeed, most employees ceased to work at the plant in December 2008. But the plant was still in operation until April 2009. So, Ryan did not lie at all. Nor did he tell the entire truth - that the plant would have closed in any event. Obama was simply bloviating in order to secure votes.
Obama did not overtly promise to keep the plant open. But neither did he do anything to keep it open. He did not lie, either - he simply misled workers by implying he could keep it open in order to win their votes.
Ryan could have made the point better, using better choice of words, but he did not lie. Obama did not lie, either - he just conned people into thinking he would save them.