It occurred to Tom to wonder whether Jane was quite trustworthy. Not that he thought she was in any way politically active.

参考译文：汤姆突然想到简是不是值得信任，并不是说他认为简有些热衷于政治。

解析：not that 从句 意思“倒不是；并不是说”相当于“you are not suggesting sth” 

She hasn't written─not that she said she would.

她还没写信来——倒不是她说过她要写。

'Not that it matters,' said Tench indifferently.

“那并不要紧,”坦奇漠然地说。

Janice had lost some weight, not that it mattered.

珍妮丝瘦了一些，这并不重要。(用法同上）

not that (用于引出否定从句)并不是说，并非 的英文解释如下

You use not that to introduce a negative clause that contradicts something that the previous statement implies. 

His death took me a year to get over; not that you're ever really over it.我用一年时间慢慢接受了他去世的事实，但其实你永远不会真正完全忘掉。