The existential reading of Scripture can be challenged on existential grounds. The issue is not, of course, with there being existential applications of Scripture, or believing that Scripture is profitable for personal growth in a variety of ways. But using Scripture as a resource allows people to avoid confronting its claims to authority and historicity. And that avoidance of a significant issue is inauthentic.
On existential principles, then, they should have the clarity to ask the questions: 1) Does Scripture state historical facts, and what do those facts imply, if so? 2) Given Scripture's claims to authority, does rejecting the authority while appropriating the wisdom, inevitably introduce hermeneutical distortion? In other words, by reading Scripture against its own grain, possibly what they are finding in Scripture is what they brought to it, rather than what it contains.
That should at least cause them to be honest in describing what they are doing: Scripture may be a catalyst for change, but on their premises it is one with limited corrective power (because it can't challenge their existentialist assumptions).