....I wonder if a computer can truly be random.
Well yes and no. In fact, even a set of three dice cannot be "truly" random since dice aren't perfectly made they will ever-so-slightly favor one side or another. With most (well made) dice, the amount of bias is insignificant.
Similarly, to my knowledge, computers also can't make simulated dice rolls 100 percent random. However, they have improved over the years where they are even better than physical dice. Any bias is insignificant as to be immeasurable. Of course, if the programming is weak as apparently in the case of Spotify, this isn't true. However, in their case true randomness (or as close as you can get to it) isn't that important.* You can bet SOM has a good random number generator.
*This isn't necessarily a bad thing for a music program. If it were truly random, you might get the same song playing in a row or just a song or two apart. They might have a sequence of numbers they use to simulate randomness so this doesn't happen, which would also explain getting the same sequence of songs. I am just guessing however.