I believe you are taking that sentence out of context. Try looking at the BIG picture and see what was happening with coins 1964-1967.
My understanding is that with the impending costs of silver and the country going off of the silver standard for coins, massive amounts of coins WERE being taken off the market. The end result was a coin shortage in 1964. So much of a shortage that production of some 1964 silver coins was extended into 1966. Kennedy Half Dollars comes to mind as well as Washington Quarters. The Treasury Departments response to these shortages was to "eliminate" mint marks from all coins and divert special interest coin production equipment (i.e. proofs which required special handling) into regular production. The SMS coins were produced to satisfy the uproar heard in Washington by coin collectors who were adamant that they were not responsible for the current shortages as much as non-collecting speculators. It was a fairly weird time back then.
If you already know what happened then what Hobo and 19 said explains it. But if you don't know what happened then it kinda does but kinda doesn't. Short and sweet - people in the govt. got it into their heads that the coin shortage was being caused purely by coin collectors. So to put a stop to that the govt. thought that by removing mint marks from the coins that that would stop coin collectors from removing the coins from circulation. But, as 19 said, coin collectors were not the problem, ordinary people were the problem. Everybody was removing coins from circulation because they contained silver and they knew the new coins would not contain silver. It was a problem the govt. brought upon themselves. But the govt., being the govt., never admits to making mistakes. Instead, they always blame somebody else, and in this case they blamed coin collectors. You see, since 1947 they had produced annual Mint Sets and annual Proof sets. The govt. thought those sets satisfied coin collectors. So to continue satisfying coin collectors, and to help the mint be able to ramp up production of circulation coins to help alleviate the coin shortage, they decided to stop making Proof sets and Mint Sets, and instead just produce 1 set, the SMS sets, to replace the previous 2 sets. The SMS sets were to be a combination, a Proof set and a Mint set all in one set. This meant that the time and the materials that the mint previously spent producing those 2 sets would be cut in half. And that the mint would therefore be able to use that time and those materials to produce more coins for circulation to solve the shortage issue. Of course it didn't work because that wasn't the problem. But it took 3 years for the mint to catch up and produce enough coins to replace the silver coins that were being removed from circulation by virtually everybody. And once they did that, then they went back to producing annual Mint Sets and Proof sets.
