Op, what you read us true for most, if not all, NON-Vanguard MF vs ETFs. As Jack was alluding to, Vanguard mutual funds get the benefit of this because rather than have two separate entities and separate sets of holdings, Invesor, Admiral, ETF, Signal, Institutional, etc. are different share classes of the same overall fund. You don't see that outside Vanguard.
Basically, an ETF is more tax efficient because if you sell 10k of shares, the underlying assets do NOT need to be sold to meet your redemption. Instead, your shares of the ETF are sold to whoever is buying 10k of the same ETF. That way, the fund need not turn over any assets. If there is an imbalance of the overall underlying fund, the ETF can swap out the an equivalent amount amongst all it's holdings to an institution making a market for a holding and they provide the cash instead.
Now, with Vanguard MFs and ETFs, since they are all part of the same fund, redemptions in the MF can be passed through the ETF. Thus, all Vanguard MFs are as tax-efficient as their ETF counterparts. Other companies can't simple pass MF activity through the ETF.