Actually I have heard that term hundreds of times....the rest of that ad is cheesy, but the baby big block term, has been around Detroit for a good number of years from say 67-68 to 71-72......it was quite common.
REASON being a stock 340 in a gen 2 barracuda or duster , would blow the doors off a stock 383 - 440 from 0 to about 80...because most of the 383- 440's back then came in B body's, the very few that did come in A bodies still had issues beating the 340 because of weight and 340's flat out wound up faster.....
Stock Big blocks had to wait to catch up with the stroke...once that happened which was somewhere around 65-70 they would start to pull on the 340's and once they caught em around 80 90 the big block would pull past em like they were tied to a post!
BUT out of the HOLE....340's would jump & be gone, while the BB Bodies would sit there and melt tires.