同学你好，该知识点来自沪江网校《新概念英语1、2、3、4册连读【双旦特惠班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
是的，前半句相当于 While he was learning online的省略，省略了he和was

状语从句同时具备下列两个条件:
①主句和从句的主语一致，或从句主语为it；
②从句谓语含有be的某种形式。

从句中的主语和be动词常可省略。
Mary often helped her classmates when (she was) free.
While (you were) walking the dog, you were careless.
John won't come to your party unless (he is) invited.

也可以看看这里的解释
https://www.hjenglish.com/new/p1204503/

祝同学学习进步~