What makes it modern is how it's built and what makes web apps modern today. Its features make it modern, I am not sure why you suggest it's needlessly resource-heavy.
I'm not saying this is resource heavy, I'm saying that it's the only specific thing I can think of that "modern" implies to me (since it's very often true of things that are billed as "modern").
Maybe a better approach would be to consider how you'd try to convey the same information to a reader 50 years in the future, when "modern" may mean something very different to them. What does that actually mean, specifically? What are the good and desirable qualities the term's intended to express to the reader? It's not a good word for communication because it's both vague to begin with, and heavily abused in marketing, so people's ideas of what it might mean vary wildly—it's so lacking in specificity that it may as well be a made-up nonsense word, as far as how much information it carries.
Some things I associate with the word modern:
* New, fallible, and untested
* Trendy and fleeting
* Undocumented
* Incompatible and unsupported
* Needy
* Pet project, single author, no community
I haven't used DFlex or what it intends to replace so I can't comment on what would be a good replacement for the word 'modern', but I doubt you meant to say any of the words I listed above.
