At least based on a cursory Wikipedia glance ̣(which I'm aware is no substitute for fluency in the language, and I won't claim to have expertise by any stretch of the word), what I see and hear supports this. The ngã marking (the tilde) denotes a tonal rise in the Northern style and a fall in the Southern, and all the audio I've heard of the ngã markings here rise. It's harder for me to hear the difference with the nặng marking (the dot).
It actually surprises me a little that even though the course dedicates a lesson to the alphabet alone, it doesn't seem like there's much singular focus on the tone markings. I can understand that might be something difficult to teach in the Duolingo format, although it seems like there were some strides to address this issue with the inclusion of actual human audio recordings rather than synthetic voices.