Fi traffic still traverses the telco's network. Unless Google has special privacy clauses in place, there does not appear to be anything blocking Sprint (et al) from implementing the 'what websites are users visiting while in your store' feature mentioned in the article.

Except Telcos cannot link (time, ip) with a paid account when traffic is only passing through them.

I don't get why Google acting as a vertical monopoly in the data tracking -> advertising value chain makes it better.

Because Google doesn't sell your data to marketers and aggregators like telcos do.

But google is a marketer and aggregator. What is the difference.

That your data isn't passed around to multiple companies that you don't know, have no relationship with and you don't know their privacy policies.