What is the meaning of multicellularity?
Bacteria are, as a general rule, unicellular.  However, there are some, like the cyanobacteria which you referenced, which are kind of border line.  Wikipedia's definition of multicellularity is a very broad sweeping definition, and its statement that bacteria are unicellular is also a broad sweeping statement.
The fact is, that as a general rule, bacteria are unicellular, but at times, like you said, they can undergo filamentation.  When they do, they end up with several connected cells which are generally considered functionally independent.  In certain circumstances, however, the cells can be dependent on each other for survival, and because they didn't separate completely, it's hard to define whether we're looking at multicellularity or symbiosis.
It is generally accepted that cyaonbacteria are truly multicellular.  In order to be multicellular, an organism generally has to fulfill 5 basic requirements:

More than one cell
Cells stick together
Cells communicate
Cells are dependent on each other
Cells are differentiated

By these requirements, some cyaonbacteria technically are multicellular bacteria, defying frequent claims that all bacteria are unicellular.  That said, wikipedia isn't totally wrong, since as a general rule, bacteria are unicellular.  Cyanobacteria are an odd (debatably) exception to the rule.

Sources:

Bacteria with bodies - multicellular prokaryotes, Gould, S.E.
Multicellularity in Cyanobacteria, Paleontology Podcasts
Introduction to Cyaonbacteria, University of California Museum of Paleontology
Wikipedia: Multicellularity, Cyanobacteria, Filamentation