They've already moved to become a full forwarder, including warehousing, order consolidation and other services (or are in the progress), so I imagine this valuation is based on the market for forwarders, not brokers.

from the article: Petersen told Forbes he expects revenue of $500 million this year, yet that still makes Flexport an underdog. “There are 25 freight forwarders that each do more than $1 billion in revenue a year,” he said.