The issue is a bit more complex than just weighted vs. semi-weighted.
Is a Semi - Weighted keyboard good enough to play classical music?
Yes, it's perfectly possible to play classical music on a semi-weighted keyboard. Assuming it is velocity-sensitive, then you can express the dynamics of the piece you are playing. If you plan to perform on the same keyboard as you are learning on, then I don't see any issue.
Is it true that playing on a semi-weighted keyboard doesn't mean that you will do well on a classical piano?
You will have a hard time controlling the dynamics if you are not used to the piano you are playing on. If you want to perform on a classical piano, ideally you would practice on the same piano or something with a similar keybed.
I see that many answers make it seem like there are only a few types of keys: semi-weighted and fully-weighted. That's an oversimplification. There is at least one more type to consider, and that is the hammer-action keybed. You can find these on some synthesizers and electric pianos (especially the more expensive ones) as well.
A big difference between keybeds with and without hammer action is that the ones without hammer action measure the velocity of the key before it is fully depressed; they don't measure the force with which you pressed the key. With a accoustic piano or one with proper hammer action, it is the force (or perhaps more accurate, the total energy accumulated in the hammer when it strikes) that matters. This means that you have to play quite differently to get the same resulting dynamics.
But even that is a simplification, because there is a huge variation in the design of keybeds. I've once had the pleasure to play on a accoustic grand piano that had lighter keys than some semi-weighted synthesizer keys I played. If you had practiced on a grand piano with much heavier keys, then performing on the one with the much lighter keys would also have gone wrong.
Personally I think it's valuable to practice on a variety of keybeds, as this will make it easier for you to adapt when playing on a piano you are unfamiliar with.