At a high level, some of this seemed plausible at first, as this wouldn't be the first time that a handset maker throttled devices via some on-device setting at bequest of a network operator. If you've been with us long enough you'll probably remember the case of the HTC Inspire 4G and Atrix 4G, two handsets which AT&T disabled HSUPA on, and later re-enabled with an update. Later there was the AT&T Nexus S which also had its HSDPA and HSUPA categories limited via build.prop. Thankfully this is not the case currently with any iOS devices.
The details are a bit technical but some of the key allegations were the Apple was limited the iPhone 5 to HSDPA Category 10 on AT&T and had specified throttling settings for the LTE network.
AnandTech notes that it's actually only the iPhone 4S that is limited to HSDPA Category 10 since it uses a Qualcomm MDM6600 chip which is only capable of up to Category 10 on the downlink. The settings for the iPhone 5 is actually in the file 'overrides_N41_N42.plist' which may have been overlooked by the original poster. In that file the correct HSDPA Category 24 (64QAM dual carrier - 42 mbps) setting is found.
In regards to the throttling settings, they aren't for data speeds rather they are throttles that prevent the phone from continually trying to reattach to an LTE network in case of error. They basically prevent your device from wasting battery life endlessly when there is a network issue. Additionally, it prevents an overload that could be caused when too many devices are retrying to connect too fast.
You can read a lot more technical data at the link below but the reality is that Apple isn't limiting their devices.