No, if you look at the actual confirmed dates, the mekong catfish is not the longest one. Giant freshwater stingrays are undoubtly longer, as well as chinese paddlefish, even if you only count confirmed records and not the old bigfishstories. The largest wels are also confirmed to have reached a bit more length than the largest confirmed mekong catfish, even if this are mainly some centimetres. It is possibly the heaviest, but surely not the longest. And there are actually no good reasons to think that there are freaks which are much larger than the largest one which were ever caught. At the best, they could be a little bit bigger, but not by far.