It depends on exactly what the claim is, but it's certainly not the "longest bridge in the world".
Wikipedia's list of longest bridges, lists Danyang–Kunshan Grand Bridge as the Guinness Records holder with total length of 164km.
Guinness' Longest Road Bridge is Bang Na Expressway with length of 54km.
The Sheikh Jaber Al-Ahmad Al-Sabah Causeway is "only" 48km long, but it's largely over water, so it might qualify as the longest bridge over water depending on how it's measured.
The current longest aggregate bridge over water is Hong Kong-Zhuhai-Macau bridge with total length of 48.3km, which is very close to what the Shaikh Jaber bridge is reported to be.
If the Sheikh is a continuous bridge, it will probably beat the current longest continuous bridge over water, Lake Pontchartrain Causeway (38km).
It's possible that the news reports are incorrectly referring to it as the "longest bridge" when they really mean it will be the longest continuous bridge over water.