Well, to be fair, one could say that you can't learn to play piano on anything
but a real piano. After all, even a weighted digital keyboard isn't going to give you the resonance and harmonics of a baby grand! And where are the rest of the 88 keys?!
Bollocks.
A low-end digital keyboard is a fine substitute for a beginner if you can't afford better.
Is it perfect? Far from it.
Will you eventually find yourself limited by it? Certainly.
But can you learn most of the fundamentals on it? Without a doubt.