This distinction is so much so that where the latter is lacking, as it is for these unlikely gardeners, the former becomes all the more urgent.
这种区别是如此之大,以至于在后者缺乏的情况下——就像对于这些不太可能的园丁们一样——前者变得更加紧迫。
that引导的结果状语从句,说明distinction显著的结果。从句主干为"the former becomes all the more urgent"(前者变得更加紧迫),前面where引导的是条件状语从句,中间插入as引导的方式状语从句。整体表达"在后者缺失的情况下,前者变得更加紧迫"。