Did you miss the part from Tanjiro where he refuted Rengoku Senior by saying that the mark on his head is pure coincidence? Others may think Tanjiro is a chosen one, and while he certainly does appear to be the secret heir to Sun Breathing, but they've set this up all very nicely to make Tanjiro very similar to Rengoku.
The burn mark from childhood was always there from episode 1. We saw the moment it got to its present state. We knew that Rengoku wasn't favored and had to teach himself after his mother died and his dad became a deadbeat. And Rengoku thought he was a loser too, but everyone loved him. Because he took after his mother, with the hard work ethic and infinite kindness she instilled in him. The same "stuff" that Tanjiro has always exhibited.
Tanjiro followed up the reminder-flashback with his own line that he's not special. He just works hard like all the Good Boys do. He's one of those heroes who is just going to try even though he will probably die (and the plot will of course spare him for that display of actual virtue because this is a shonen and not a seinen).
They have sort of explained this. We're quite a few years into the series timeline now, because all the training montages are serious timeskips. Two years on the mountain to learn water breathing. A few months more traveling to the first test, several months each time there's another healing and more training montage after one of the big fights, because they heal at normal human rates.And then Nezuko gets a power-up in the most random way. She's a demon. She can't train her body, it stays the same forever. All the power is given to her from the beginning (unless she started consuming human bodies, but she doesn't do that). Why activate this power now and not in countless earlier life-threatening battles? Eh.
It was nice to watch, no question. But it didn't feel "earned".
Nezuko has slept in her box, or in a room, the entire time she's not shown on screen (or shown awake in her box). The elders of the series have somewhat implied that in not drinking blood, and sleeping the whole time, Nezuko's body has been up to something that none of them have ever seen before. Which is because Muzan normally takes them in right away (like he re-explained to the audience through Daki) and gets them onto murdering people.
It has been implied that her hibernations are why she has displayed such absurd strength despite never taking in a single drop of human blood, or that her regeneration is far above what it should be (not the first set of legs she has lost if you remember). She compresses her size and just accumulates power. Maybe through Muzan's blood all by itself?
But I'm with you on the Nezuko having this horn power-up and the vine markings (similar to Daki's flower marks and other Upper ranks that they mentioned at one point). That wasn't earned at all. Maybe we'll see something next time to explain any of it, but I dunno.
Did Muzan go all out and inject her with an Upper Rank quantity of blood from the start or something? Or maybe the setup they've worked in here is that indeed, Tanjiro did not inherit the 'chosen one status' that makes a forehead mark. Because Nezuko did. It could explain her burning blood.
Bit of a retcon with the younger brother saying how scary Nezuko can get when she goes into Anego Mode. So yeah, explained why her demon form is mountain ogre, but wasn't earned like Tanjiro's parts were.