They're only comparable to 90s supercomputers in the sense of compute power.
But when it comes to productivity they are fundamentally limited by their physical size and limited input/output real estate vs a desktop or laptop, or even 90s supercomputer with a 25" CRT and big clicky keyboard.
Even laptop keyboards use nearly full size keys, just as piano keys come in a pretty standard size, regardless of the size of the piano.
Programming on a phone directly would be like cooking for a banquet on a single camp stove instead of a full kitchen. You could do it, but why?
And sure, you can hook up a keyboard, mouse, and external monitor to some Android phones, but at that point, it's hardly a phone anymore.
I am typing this on an old iPad, and productivity level on it could be at least on par with my 1995 rig. Phone, not so much, without a larger display.