> W and Z bosons, photons, etc have fixed masses, charges, interaction strengths with other particles.

But you can form a continuous set of linear combinations of these things, just as you can with gluons. Indeed, what the article calls W and Z bosons (and photons) are just such linear combinations--the ones that appear in the low energy limit after the electroweak phase transition occurs. Before that phase transition, different linear combinations (i.e., a different basis of the electroweak vector space) are the ones that naturally appear. So saying that there are two W, one Z, and one photon is really counting basis vectors in the electroweak vector space, just as saying there are 8 gluons is really counting basis vectors in the gluon sector of the strong interaction vector space.

In a hypothetical scenario where we were inventing the standard model in the first 10^-11 seconds after the big bang, you're right there would be an analogy there. But in that scenario, our standard model would say there was one electroweak particle, not that there were 8 gluons.

In our own universe, the fact that electroweak symmetry breaks ensures there are 4 electroweak particles and not other combinations. There's no corresponding thing to contain gluons to individual particles, you'd need laws of physics we don't have to add that constraint.