Using 莫 in Song dynasty Chinese
問﹒普萬物﹒莫是以心周遍而無私﹒否


i would interpret it as :

[someone] asked (問): “[about] generalising (普) everything (萬物), can it be that (莫 —> 莫非) [the universal] is using (是以) [its] heart (心) to all over (周遍) and (而) without (無) prejudice (私); isn’t it (否)?


  How to understand this 莫 ?


back to your question, i would say it means “莫非“ (roughly “can it be that”). the asker wanted to ask, but without confidence that his statement is correct, so he inserted the “莫” before the verse “是以心周遍而無私﹒否”; as a safe guard.

clearly, you have fun in reading “朱子語類” :)