maybe I’m misunderstanding it too, but I feel like if you don’t get a small negative number or 0 on this loudness penalty site and you are targeting one of these services, realistically, you are putting the music you are mastering at a “disadvantage” (as if you play stuff next to each other, music that is played louder is more “engaging” than music that is played quietly…same reason why when you A/B changes it’s very important to do so at the same volume to actually be able to hear the difference and not just pick the louder one) , as some/(all?) of these services do not turn things up to reach their loudness target, they just turn stuff down if it’s above.
so in essence, I think there still is a reason to master “loudly” (enough), it’s just that it’s not based on peak limiting anymore, it’s based on a more complex algorithm.
for some music/targets this is probably not a big deal at all, as “loudness” is not the most important thing, but I think it’s important to understand what is going to happen so you can make that decision.
also, if someone knows more and sees some fault in my logic here, please let me know