Nah they're trying to be nice. Honestly, to me, it reads much more like they aren't even registering that they are a potential romantic partner who is interested in them. It also makes it seem like they were using the guy who made the playlist, which is unlikely to be the actual intent. A more straightforward rejection would be preferable. Being ignored hurts more than being rejected imo. That being said I don't see how you could reject advances in a polite way in this situation.
Btw, have I mentioned how much I love taking funny Internet memes seriously?
Edit: I made an admittedly weird argument that wasn't an argumented well on my part (should have been more about how it can be interpreted differently rather than it being "not nice," whatever that means.) However I'm not a coward, so I'll keep this up. But the argument below is kinda mental, with nothing much to be gleaned, so be warned .
Believe it or not, women are much more likely than men to have platonic friends of the "It took me a couple hours to make this Playlist for you" caliber.
Women aren't stupid, they know what an advance looks like, but they're also not afraid of just being friends like this. Yeah, she was definitely drawing a boundary by mentioning her boyfriend, but she was also saying "I appreciate you in a platonic way" and not just burning that bridge by telling him to fuck off.
Another reason lots of women approach this way is because outright rejection can be actually dangerous, if they see that person in their day to day life.
I personally think dropping the boyfriend card is the right move here if she's not interested. If he knew about her boyfriend, it's his fault for trying to get in there. If he didn't, she's politely letting him know she's taken. Either way, the onus is on him to read the situation, not her to tiptoe around his feelings, and she still left the door open for friendship if he wants that.
It may not seem like it if you identify with the guy being spurned, but she is being nice.
I believe it is implying that the sender made a playlist specifically to attract the attention of recipient in the hopes of dating said recipient. Instead, the recipient listens to the playlist with a (new?) boyfriend, thus excluding sender and senders' efforts.
I'm literally using Spotify right now and the "heart" button still exists... It's just a plus inside a circle now. It only does more than like a song if you press it twice.