This is true at all. Explosive spalling is a very real thing and you can see many examples of this online. Cracking is a minor manifestation but it can be much more violent.
(I'm assuming your first sentence is missing an "isn't")
We're talking about mortar here, not concrete. I.e. the "paste" that's put between bricks and other stone work to "glue" it together.
But of course normal bricks are also subject to spalling, but it's very rare that they'll Gail catastrophically, mostly you'll get a crack or two.
Europe's full of old brick houses that cone chimneys that were made before using specialized materials for them was mandated.
You can safely build such structures, they just require building and usage techniques that go along with them, e.g. using a double layer of brick, or gradually heating up the chimney to get the humidity out of the bricks and mortar.
It's mandated in many places (and for good reason) for indoor fireplaces, chimneys etc.
That's because regular mortar will crack when subjected to roaring fires, and you don't want the heat, fumes or fire itself to escape the chimney.
But e.g. for a fire pit it's usually no big deal, as the outside of the pit is presumably soil, or the open air.