Brunson scoring at will against a papier-mâché defense
I think it is more good offense than bad defense. Brunson is a tough guy to matchup with. Indy's starting combo guards do not match up well with him. Only McConnell and Nesmith do.
Thankfully BOS has D White and Jrue who can both matchup very well with Brunson. Haliburton and Nembhard do not have enough muscle to fight with him. White and Jrue do. I expect Jaylen will be the 3rd option to defend Brunson. Similar to Nesmith.
Nembhard is a good defender. Just a bad matchup for him. Brunson is making him look silly.
Haliburton has always been more of an active team defender than a one-on-one defender. He can't guard anyone one-on-one. Small guards, big guards, wings. None of them. Uses his length well in team defense to help defensively that way but struggles in any one-on-one situation.
McConnell does well because he can match Brunson's speed and physicality. Nesmith does well because he is physical and much bigger than Brunson ... and Brunson is slow so he struggles to find a speed mismatch like most PGs can do in such a situation.