There is a pretty big difference between a charitable non-profit organization and a proprietary music streaming service.
This is, apart from the utility that it brings, advertisement for the operator:
> Wikimedia recognizes the user is on that operator's network and serves a banner on the top of the page indicating free data courtesy of their mobile operator, which reinforces a positive brand experience for the operator. [0]
It's not inconceivable that the ISP would do similar deals with $music_streaming_service. Maybe for some payment.
Just because Wikimedia is not seeking profits or even looking to somehow "stay ahead" with this deal, that doesn't mean it isn't against neutrality.