The "get back to you" response is not entirely without rationale, because the press secretary might not yet know the answer or wants to make sure he or she has the most up-to-date response. But when you never get back to all the people you promised, then when do those broken promises become a string of lies?
And when this becomes chronic, it turns from delaying into outright stonewalling. Remember when Trump constantly whined about President Obama's alleged lack of transparency and then later promised to be the most transparent President in history?
Well that of course turned out to be an outright lie. Constant stonewalling and rampant secrecy do not transparency make.
At some point, a promise to "get back to you" automatically converts into a lie. "Get back to you" has an inherent implication of "sooner rather than later." Most everyone agrees it is not the same as "I will never get back to you."
Prior to the press secretaries' employment, Trump regularly made a practice of promising to "get back to" something and then never bothering to do so. He still does this somewhat regularly.
Given many if not most of these promises were never fulfilled, they became outright lies.
One example that comes to mind is Trump's promise to answer soon all questions about the clouds over his wife's immigration history (link here). Ten months ago, Trump promised that news conference in a few weeks. Ten months later, it has yet to happen. I think we can assume now that was a lie.
The greatest example, of course, is Trump's notorious promise to release his tax returns. He first made that promise more than three years ago (link here) and repeated it several times more than year. But more than three years later, still no tax returns.
Only a hopeless optimist would see that as anything other than a bold-faced lie.