A sine wave puts out energy at just one frequency. If your intent is letting a bass speaker go up in flames before a tweeter does, a sine addressing just the bass speaker is the right tool. A rectangular wave of the same amplitude, however, contains just the same amount of energy at the fundamental frequency. So with a single speaker, it's at least as capable of blowing out the speaker.
The reason it doesn't do that is that you won't play it for 10 seconds since it hurts, cranking out ugly noise at frequencies your hearing is sensitive at. In contrast, if you play a low frequency sine at large volume over laptop speakers, it sounds comparatively benign. Particularly if you play it at frequencies where laptop speakers don't even manage to convert it into acoustical energy because they are too small.
The largest killer for large speakers is DC which is the lowest in frequency you can have, transfers no acoustic energy at all and heats the speaker coil without any venting by movement. Now laptop speakers don't have much of a venting effect anyway, so DC and low frequencies are similarly harmful, and when you work them at frequencies it does not significantly convey to your hearing, it will be similarly unnoticeable. The laptop circuitry actually tends to provide protection from DC (via decoupling RC parts) but not so from low frequencies ostensibly in hearing range.
So I'm pretty sure you could have taken out your speakers using rectangular waves equally well or better. It's just that you would have stopped before they blew out because you would have heard that you were doing something stupid. Sine waves are not more cruel than rectangular waves of comparable amplitude or other stuff, they are just stealthier, particularly at low frequencies.
If it's a consolation to you, rectangular waves excel at taking out tweeters in multi-way speakers even when their fundamental is still in the range of a bass speaker. But for single-way speakers (or if you target specific speakers of a multi-way setup), sine is the way to focus your energy.