What did Al-Albani mean by his statement that legal jurists do not support their claims with the Sunnah, nor do they have knowledge of it, but rather dispense with that by relying on their own opinions?

The essence of the discourse is the condemnation of blind imitation and the following of whims. It criticizes jurists who have deviated from the foundational principles and no longer support their statements with the Sunnah (definition) due to their ignorance of it. Instead, they rely on their own opinions and what immediate interests dictate to them, which is a door to temptation and misguidance. The early generations (Salaf) condemned every opinion not based on the Qur'an and Sunnah. It was reported from Umar ibn al-Khattab: "Beware of the people of opinion, for they are enemies of the Sunnah. The Ahadith became too difficult for them to memorize, so they resorted to opinion, and thus they went astray and led others astray." Ibn Mas'ud said: "There is no year except that the one after it is worse than it... but it is due to the departure of your best people and your scholars, then a people will emerge who measure matters by their own opinion, and thus Islam will be demolished and impaired."