你的理解错误。

分号后可以理解为省略形式的并列分句：What they offered was classic French-restaurant cuisine, short menus cooked fresh à la minute（法语“非常快”）; (what they offered) was not, as was the custom even in high-class British restaurants then, dishes reheated from frozen or cooked far in advance.

as为比较分句，意为“像高档英国餐厅当时习惯做的那样”，由于该as分句在not的否定范围之内，所以其意义为，不像高档英国餐厅当时习惯作的那样。

全句意思为：

他们所提供的是经典法国餐厅佳肴，菜品不多但都是现场烹饪，而不是像高档英国餐厅当时习惯做的那样，将冰冻食品重新加热或者提前很久就烹饪好了食物。