Chris is very much a trusted spokesman for Microsoft (and Nokia in the past) and the article is on the official Conversations site. Here are a couple of the quotes from the full article: (my emphases)
For example, you can get the budget-friendly Lumia 435, the selfie-happy Lumia 735, or the gorgeous gold-edition Lumia 930–and you will be able to upgrade to Windows 10 as it becomes available, starting this summer. It’s the best of both worlds: amazing hardware right now, with a VIP pass to killer new software later this year.
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 operating system, 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.
All of this shouldn't come as a surprise, given what I've been saying in the podcast and theorising in print now for a month or two. Microsoft's aim has clearly been to make sure there are phones even at the low end that have a high enough spec to run Windows 10.
The wording is ambiguous, of course. 'Lumia phones' could refer to the range of devices or to the installed base of tens of millions of phones. My guess is that it refers to the range and that the likes of the Lumia 530 and perhaps even the 520 and 635, plus several third party designs (including HTC), will all be left on Windows Phone 8.1 Update 1. The RAM requirement is probably less stringent than that for internal space, mind you. However efficient the code is at running on a Snapdragon chip, you can bet that there will be more of it and 4GB devices will be squeezed out.
Note also that 'majority' doesn't even imply 'most', it simply means 'significantly more than half'. I'm still expecting that big sellers like the 520 and 630/635 may be left on 8.1 Update 1 too, or at the very least presented with a cut-down experience on '10', because of the 512MB of RAM. Of course, this probably won't matter to most 520/630 owners, who are very unlikely to be tech enthusiasts and won't even be aware of any further upgrade possibilities on the horizon or under-the-hood technical limitations.
Comments welcome - I know that some people disagree with my theories on this matter - I'll put it all to the Oracle (Rafe) in the podcast tonight!