The word "quasi-coordinator" means "a phrase that resembles a coordinator" (conjunction). It looks like a coordinating conjunction but it functions more like a preposition. This is important because nouns that are linked by these phrases do not become plural subjects if the subject noun is singular.
If you look at Fowler's example you will see "along with many other companies in the UK" is -- a singular verb.
If you replace "along with" with "and" you would need are -- a plural verb.
The President, accompanied by his Secretary of State, is entering the meeting room.
The President and his Secretary of State are entering the meeting room.
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