I was wondering about this as well... I wonder if they were initially worried that having the Superman comic AND regular Superman-covered Action would be too much...?
But thinking about it further... look at what happened around Jan-Mar 1939: The newspaper strip launched, they moved to squash the competition with DC v Bruns, and they were prepping for the Superman comic -- and of course, by the end of the year there were monthly Superman Action covers. So that fits with the all sail and no anchor quote for fall '38. They were preparing to blow things up in '39, and they did.
Keep this quote from David Bachman in mind...
Superman #1 was not, in fact, Superman #1. It bore no number and no date (except a copy-right date), either on the cover, or in the indicia. More significantly, it bore no notice of second-class mail permit in its indicia, an item that must be carried by any publication that will be published periodically and sent to subscribers via second-class mail. The house ad in Action Comics #13 referred to it as "a big complete Superman book" rather than as a new title. And the house ad for Superman #2 in Action Comics #17 stated, "Your overwhelming approval of the first magazine has prompted us to publish this second one...." Cumulatively, these items convince me that the publication we refer to as Superman #1 was actually intended to be a one-shot publication. Its "overwhelming" success, then, apparently prompted two reactions by its publisher: (1) publication of a second printing, and (2) a decision to initiate a new quarterly title featuring only Superman stories.
This is the first time, I've seen this or heard about it... that is very cool!
I never thought about Superman #1 NOT being #1.