It’s an accurate statement for a couple of reasons:

• Large number of U-boats available: The main limiting factor for the U-boat campaign’s success in the first half of WW2 is that there simply weren’t enough boats. Admiral Dönitz became convinced during the interwar years that the UK could be defeated by U-boats alone if Germany could field a fleet of 300 boats. When war began in 1939, however, Dönitz had…56. His fleet did not reach 300 until 1943 when the tide was turning against them.

During WW1, Germany’s 1915 “unrestricted” submarine campaign failed to significantly impact Britain largely because Germany only possessed a few dozen U-boats capable of Atlantic operations. This had changed by 1917 when Germany launched another unrestricted campaign. By this time, the Kaiserliche Marine had about 100 U-boats. Consequently, the first half of 1917 was the closest in either war that Britain came to total defeat.

• Inadequacy of WW1 anti-submarine technology: Allied technology ultimately defeated the U-boats during WW2 by stripping both their underwater invisibility and their ability to operate freely on the surface. By 1943, traveling on the surface meant detection by radar and being spotted and attacked by aircraft. Traveling submerged, where they were much slower, was not always an option due to battery limitations. When they did, they were often detected by sonar and then attacked with depth charges. Even trying to hide in an Allied destroyer’s sonar blind spot only put a U-boat directly in the killzone for the “Hedgehog” mortar system. The Allies could also determine a U-boat’s location by triangulating their radio messages. The U-boats essentially had nowhere to hide, and that’s not even mentioning that Allied cryptographers had broken their codes.

This was not the case in WW1, however. Sonar and radar were not yet in service. Depth charges did not appear until 1916 and, without sonar, they were of somewhat limited use anyway. Allies aircraft also lacked both the quantity and quality compared to the following war. A submerged U-boat during WW1 was therefore both invisible and nearly impervious to attack. Moreover, anti-submarine doctrine and experience was also far less mature than in WW2. The Allies ultimately avoided catastrophe in 1917-1918 by implementing a convoy system, but they never really tactically defeated the U-boat during that war.

tl;dr the 1917 U-boat campaign came closer to defeating Britain than any point during WW2 because Germany had a large U-boat fleet to employ and the Allies lacked the technology to defeat them on a tactical level.