I'm not sure what math you're basing this on, but it's far from correct. They include slightly less transfer in the instances, but their overage pricing is about 10% of DO and their instances are so much cheaper that you'll be paying much less even after adding a few TB.

For example compare the CPX11 to the cheapest similar offering from DO:

CPX11: 2vCPU, 2GB, 40GB storage (NVMe), 1TB transfer, ~$5 per month

DO Basic: 2vCPU, 2GB, 60GB storage (non-NVMe), 3TB transfer, $18 per month

Add 2TB more transfer at Hetzner and you're at ~$7 per month. Still a bit less storage though, let's also compare it to the CPX21:

CPX21: 3vCPU, 4GB, 80GB storage (NVMe), 2TB transfer, ~$9.50 per month. Add 1TB of transfer and you get more of everything for ~$10.50, ~40% less than at DO. And bandwidth overages at DO are about 10x as expensive.

We can also compare a higher tier:

CPX41: 8vCPU, 16GB, 240GB storage (NVMe), 4TB transfer, ~$32 per month

CPX51: 16vCPU, 32GB, 360GB storage (NVMe), 5TB transfer, ~$63.50 per month

DO Basic: 8vCPU, 16GB, 320GB storage (non-NVMe), 6TB transfer, $96 per month