The fundamental principle makes this clear: Is the ISP providing competitive advantage to some apps/services/websites, either through pricing or QoS? Zero-rating does violate this.

But that's not the fundamental principle they advocate for.

The letter mentions "to discriminate both technically and financially against Internet companies and to impose new tolls on them."

No technical limitations are placed on internet companies with free basics, they just cost more to the consumer. And as the company isn't charged more to be on free basics, there isn't financial discrimination against the company.

Strictly speaking, that doesn't prohibit zero rating.

I'd be more interested to see a response to the studies that Facebook quoted. Are they taken out of context? If not, they do seem to agree that free basics would help.