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.
That was the 1st InfluxDB storage engine, things have evolved in the intervening years. The latest InfluxDB design is actually quite similar to the latest Prometheus design, what's different is our approaches to reliability and clustering.
It's presently looking like Influx might once again be an option for long term storage for Prometheus.
That was my impression after the influxdb storage engine rewrite; I'm guessing the similarities are not a coincidence :)
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).
It's presently looking like Influx might once again be an option for long term storage for Prometheus.