If you mean up the most games before losing a pennant or division, that might be right. I can't find any other team up as many as 14 and ending up out of first place.
If you mean the most sudden collapse at the end of a season, you would be definitely be correct, if we do end up blowing it. The gold standard has always been the 1951 Dodgers, who were up 13 on August 11 with 47 games left on the schedule before coughing up the pennant (or having it stolen from them) by Bobby Thomson and the Giants. Entire books have been written about that collapse (in part because they were the Brooklyn Dodgers, natch).
By contrast, the Tigers were up 11-1/2 on August 23 with 31 games left on the schedule. A slightly smaller number than 13, but way fewer games to blow it in. So yeah, I believe we would supplant those Dodgers as the worst late-season collapse ever.