You've turned that estate into 2 silvers - and a trader. In short, all you've done is obtained a second silver buy, and that turn where you drew a trader instead of just a silver to do that was probably ruined in the process.
That's not all though, you've also got rid of two estates in the process, which is pretty significant in my opinion.
One Estate.
Your deck sans Trader Buy: 7 Copper, 3 Estates, whatever else you bought.
Your deck with Trader and one use on an Estate: 7 Copper, 2 Estates, 1 Trader, 2 Silvers, whatever else you bought.
His point is that you still have 3 cards with no buying power (2 Estates, 1 Trader). You have 2 Silvers, but if you'd bought a Silver instead of the Trader, you'd have 1 Silver. And on the turn that you exploded the Estate, you had an effectively 3 card hand at best (since you had two cards -- Trader and Estate) with no buying power.
Or, here, look at it this way. Compare a pure Big Money strategy to BM + 1 Trader.
Opening buys: Silver/Silver vs. Silver Trader.
Okay, after the first reshuffle, BM has 7xC,2xS,3xE while Trader has 7xC,1xS,3xE,1xT. We agree, I hope, that on the first reshuffle, the BM has a strictly superior deck in terms of buying power? One of its hands will be worth $2 more than the equivalent Trader hand.
Assume that the Trader deck successfully explodes an Estate after the first reshuffle. Now, what did it buy that turn? Its remaining three cards could have been worth anywhere from $1 (C,E,E) to $4 (C,C,S). So it buys either a Silver ($3 or $4) or nothing ($1 or $2).
We assume that the two decks had identical shuffle-luck. So on that same hand, the BM deck had $3->$6 in money. It is guaranteed a Silver, and may have a Gold.
Let's assume that both decks buy a Silver with their "other" hand before the second reshuffle.
So after the second reshuffle, if both decks bought a Silver on the "Trader" hand (which is the best result for the Trader deck), then the BM deck has 7xC,4xS,3xE, while the Trader deck has 7xC,5xS,2xE,1xT. In which case it has the same number of no-buying-power cards, but one more Silver than the BMU deck. It is slightly superior.
However, that's the best case for the Trader deck. It's possible that this is the matchup:
BM: 7xC,3xS,1xG,3xE
Trader: 7xC,5xS,2xE,1xT
In which case the BM deck is probably better.
And it's possible that this is the matchup:
BM: 7xC,4xS,3xE
Trader: 7xC,4xS,2xE,1xT
In which case the decks (through the next reshuffle) are exactly the same (same Treasures, both have 3 cards with no buying power).
And it is ALSO possible that you drew the Trader with 4xC, meaning you still have 3 Estates.
And in the next shuffle, it is quite possible, and indeed getting fairly likely, that the Trader won't hit your second Estate.
Long story short: there are a lot of failure modes for Trader, and the success mode is pretty mild. It might beat straight Big Money, but Single Trader is not one of the better -- or even one of the kind of okay -- single-card Big Money decks. Single Thief beats Big Money too, but we don't think Thief is a good card.