I've not read McDonald's book, so cannot answer with exactitude:

But the usual sense that historians talk about the myth of "triangular trade" is twofold:

First, the notion of 'triangle trade' in earlier decades sometimes taught (erroneously) that it was the same ship that made the circuit--i.e. the good ship "Bounteous" would pick up raw materials (like timber) in Boston, ship them across the North Atlantic to Bristol, then pick up cloth and rum and sail down to West Africa; trade them for slaves to bring to the United States... the ship itself sailing in one big triangle. This is indeed a myth (and one that won't die). For instance, slave trade ships were very specialized vessels--very fast, to minimize transit time, and manned with crews accustomed to controlling enslaved peoples. Whereas a ship that brings timber from the colonies would be slow, bulky, and with a much smaller (and less brutal) crew. (My guess is this is the 'myth' that McDonald is referring to, as he talks about monopolies with slave trading, meaning that no johnny-come-lately rum smuggler from Salem was going to be allowed to import slaves... but no historians really believed this ever happened anyway.)

The triangle trade was always meant to describe a system--it's an earlier nomenclature for what historians now more frequently call The Atlantic World system, where slavery, guns and other manufactures, naval stores, and plantation-grown consumables like sugar, rum, and tobacco, were all enmeshed in a complex web of mutually dependent relationships. For instance, trade in cheap guns to West Africa facilitated and escalated slaving wars in West and Central Africa--which produced more slaves for sale to Europeans to ship to the West Indies plantations. Meanwhile, slave labor on sugar plantations produced the sugar that helped to calorically-sustain the manufacturing workforce in England that made the guns that were shipped to West Africa. And naval stores produced in New England kept the whole fleet of merchant ships running smoothly (as did the rum, produced by the slave-sugar plantations). etc.

In the second sense of the "myth" of the triangular trade is to call into question its "triangle-ness". Recent research has shown that exports on these trade networks were not unidirectional, but rather fairly complex. For instance, not only raw materials but certain manufactures were exported from the north American colonies to England. And cloth was both imported into and exported from West Africa. And a port like Bristol would offload and onload goods from pretty much everywhere, feeding them out to pretty much everywhere. (not necessarily in any sort of 'triangle', other than the fact that a triangle roughly describes the main shipping routes following prevailing winds.)

So the term "triangle trade" is increasingly replaced by the term "Atlantic World system"... or, even more accurately/specifically (and recognizing the pivotal role of slavery in all of this), the "plantation complex."

The classics here include such books as Sidney Mintz, Sweetness and Power and Curtain's Atlantic Slave Trade... and especially, Curtain's pivotal essay collection, Rise and Fall of the Plantation Complex. But there's a huge array of great recent work on all aspects, from blackness (Paul Gilroy) to slavery (Stephanie Smallwood or Marcus Rediker) to religion to ideology (Anthony Pagden)