Here is the quote from the MS rep that the OP is referring to...
"Does that mean that today’s Lumia devices can be upgraded to Windows 10?
We’re making it as easy as possible to get Windows 10. Windows 10 has been designed to run well on today’s Lumia phones. Like any upgrade to a new platform, not every phone will upgrade or support all possible Windows 10 features, and certain features and experiences will require more advanced future hardware. Our goal is for the majority of the Lumia phones running Windows Phone 8 and 8.1 to join the Windows ecosystem along with an expected hundreds of millions of PCs, tablets and other devices running the next generation of Windows."
First, the quote is from a sales exec at MS. It's a mistake to try to infer technical details from someone that is an expert in sales or marketing.
Second, his response is quite vague (again, sales guy) and it makes no mention of 512MB devices at all. The OP is making a huge leap by assuming that 512MB devices are the limitation he is referring to. It could be anything from "Hey Cortana" requiring SD800 series hardware which is already the case in WP8.1, to the faster camera support that is also only supported on current gen flagships in WP8.1, or some other new feature that would only be expected on higher end devices anyway.
Also, even if 512MB devices does turn out to be a major limitation in Windows 10, that still isn't really a good use of the term "fragmentation". This is more of a "legacy hardware" or "hardware limitation" issue than one of fragmentation. I would consider fragmentation to be incompatibilities between
current generation software and hardware. This is pretty common in Linux bases OSes and Android because their open source development allows different groups to simultaneously pull them in different and incompatible directions. Limitations caused by obsolete hardware or missing hardware is not the same situation. My laptop's Wifi chipset is too old to support Miracast even though Windows 8.1 supports Miracast. I don't consider that an example of fragmentation though.