Fact: Amazon engineered its own proprietary LTE modem.
Fact: The FireHD9 LTE has not yet been approved by the FCC because they need to test the LTE modem
Fact: the relatively high price (for Amazon) of the LTE Fire makes it likely the model will sell less than the rest of the product line, making the modem a low volume part
Fact: In the buy vs build product design debate, you buy small volume/small value parts, but build critical and/or expensive components
Fact: Amazon is generally great at identify what elements to "build" in-house and what to get off the open market
Fact: with generic LTE modems reportedly available for $41 or so, Amazon could have easily met their small FireHD9 LTE needs that way
Fact: Bezos isn't crazy
You don't invest time and money designing and building an expensive part like that ultrathin LTE modem for a low volume application. Ergo, Amazon has further uses for their LTE modem.
One such use will likely be an LTE FireHD7.
Another will almost certainly be a phone. And soon. Or, soon-ish.
The best way to maximize the impact of the Kindlephone is to keep it secret until launch. And one way to keep it secret is to get the modem certified first in a different product so that when the phone is submitted it gets rubber-stamped.
There really is a Kindle phone coming. No sooner than november, more likely early next year.