Morphological analogy derives forms by known rules; samaʿ refers to forms actually attested in authoritative usage. They relate to one another but are not interchangeable tests.
A form may appear to depart from the most common analogy yet be genuinely transmitted from Arabic speakers. Such a form is not automatically defective.
Conversely, a poet's metrical choice does not, by itself, establish that an otherwise unattested morphological form is acceptable. This is the issue developed in the textbook's next example.