Oh sure, but as a gross simplification and
by design, it's largely impossible for the user to distinguish whether a background app:
- continued to run (because it's a VoIP app, because it's still within its limit, etc.)
- was suspended, and is being resumed from memory
- was killed altogether, is literally being shown as a "screenshot", and being relaunched from scratch
In practice, there are subtle (and sometimes not-so-subtle — especially if your device is older) ways you can tell, but in theory, a background app might as well be little else but a screenshot in all three cases.
That said, regardless of how iOS internally handles background apps*, my main issue with this conversation is the idea that a 2018 iPhone is somehow not beefy enough for multitasking. That's not at all the issue. Apple just doesn't feel that a traditional overlapping windows paradigm makes a whole lot of sense on a 5-inch touchscreen UI. And even on 12 inches, they feel more comfortable with a tiled UI, with few exceptions such as picture-in-picture.
That's a deliberate design decision, not a spec weakness.
*) I don't actually currently ship one myself, so I only have limited exposure.