Considering this is an example sentence in a modern J-E dictionary, yes, I believe this is a typo. At least in standard Japanese, it should be written as 甘んじている.

[BCCWJ corpus][1] returned only one result for 甘【あまん】じる ("屈辱に甘じているのだ" in a novel published in 2002), and there may be a few other instances where old writers used 甘【あまん】じる. But let's not care about that too much.


  [1]: http://pj.ninjal.ac.jp/corpus_center/bccwj/