So I'm pretty a similar problem could exist. If to accept a proposal we only need 3 nodes, not a majority, and we manipulate network latency in arbitrarily bad ways, a similar split brain could occur. Basically, you could commit the same transaction twice under different proposal numbers.
Consensus is not about transactions, it's about agreeing on a value (a single value, strictly speaking, unlike atomic broadcast, which is about agreeing on a sequence of values, although people often identify the two). But anyway, your argument is not in any way concrete - try to come up with a counterexample. Having thought about it more, I am confident that the author's system is correct (in the sense that it always preserves agreement).
