Does this mean they will only consider those with that academic credential to have the asset qualification?
No, describing a qualification as an "asset" would indicate that they take it into consideration but do not require it. Companies that have an absolute requirement will generally make this very clear, to avoid wasting everyone's time.
Are some employers flexible enough to also consider candidates with experience in lieu of qualifications?
Yes, I have worked at several places where requirements in the job spec have been flexible. Remember that the ultimate goal of the hiring manager is (or should be) to hire someone who can do the job well - a job spec is only an attempt to codify what that person would do and be like.
In the event that there are many applications, however, those who do not meet all requirements may be culled without further consideration in order to reduce the scope of work for the hiring company.
In this case, it sounds like they do not have a hard requirement for the certificate, and would consider your application.