Harry was perfectly calm, for a simple reason.
He wasn't afraid.
It wasn't because he was confident they couldn't see through his invisibility, but because even in a head-on fight, Harry didn't think there was anything to worry about.
Before the summer holidays, after the Battle of the Ministry of Magic, Harry's assessment of himself had still been this-with the Left Hand of Chaos and Calamity, he surpassed the average wizard. He could defeat ordinary Aurors and Death Eaters, but there was still a gap between him and the very best among them.
As for an exceptional master like Voldemort, that went without saying.
But after one summer holiday, Harry's assessment of himself was now that, apart from Dumbledore and Voldemort, he would concede superiority to no one.
In theoretical magical research, of course, there was still a considerable gap between him and the professors at school. But in magical combat, Harry felt that even if he couldn't beat those professors, he could hold his own against them.
As for those a notch below them, Harry felt he could handle them however he pleased now.
Whether they were Dark Wizards or dark magical creatures, Harry now had a whole range of ways to deal with them.
It wasn't simply that Harry could improve so much in a single summer; he had acquired rather a lot over the holidays.
After absorbing Voldemort's soul fragment, he had gained some of Voldemort's magical knowledge and at least part of Slytherin's legacy. Combined with Ravenclaw's Legacy, which he already possessed, Harry's understanding of magic now surpassed that of many veteran wizards.
His time with Grindelwald had also helped him make tremendous progress.
Harry's confidence was now inflated beyond measure, and he had long wanted to find a few Dark Wizards to test his skills