When an answer recommends a product, only recommends a product, but is a relevant and seemingly genuine recommendation rather than spam, is that an acceptable answer?
I would say it may be if that recommendation is particularly relevant to the question.
For example, if the question is "How can I control the parameters more easily in realtime on my Roland JX-3P'...
then a recommendation for the PG-200 Programmer might be very relevant. (I agree with Dom's point in the comments that a good answer would describe how the particular product solves the problem, although sometimes that might be apparent from the nature of the product).
In the context of the particular question you highlighted, How can one learn to identify intervals by ear without singing?, I agree with Dom's points, but I don't think we can apply that logic to every question.