That was never true AFAIK. I recall one of the core devs did a comparison of storage engines years ago and found Influx unsuitable; that work led to the current-gen storage engine.
It's presently looking like Influx might once again be an option for long term storage for Prometheus.
I'm not sure how interested influxdb would be to the idea, given their shrewd moves towards monetization, but it would be nice if the storage engine could be developed as a component of influxdb and adopted into Prometheus(ala rocks, level, etc).