The Pronunciation Tool’s linguist note rather claims [əŋ] : https://chinesepod.com/tools/pronunciation/section/5
But its voiceovers sound in the chart beneath sound like [ɤŋ], and that is what I hear elsewhere…
Also the text above unsatisfyingly does not explain the deviation of en/eng from the ‘e’ [ɤ], instead saying they’re “roughly the same”:
For the -n and -ng final consonants, the pronunciation of e stays roughly the same, with just the addition of the consonant sounds at the end.
Really now, roughly the same? To me [ɤ] and [ə] sound completely different.
Basically it seems to me that the IPA for *eng should be corrected on that page, and the deviation of *en from e/eng should be explained as its own seemingly separate case.