Analyst is right
AT&T *LOVES* fudging the numbers. The figure AT&T calls number of "customers" is actually the total number of connections. Verizon has 94.1 million customers, but 102.2 million connections, well ahead of AT&T's 95.5 million connections.
Anyway... the reason for the IPhone running on EDGE on T-Mobile's network is T-Mobile runs their GSM/EDGE network within the 1900mhz band (which is widely supported) but they and IWireless both run their 3G in the AWS band (which is 1700mhz uplink and 2100mhz downlink.) Very few devices supports this band, and IPhone is not one of them.