Assuming the latest numbers from TC are correct, they're at annual reoccurring revenues of somewhere between from 70M-140M (depending on average per seat revenue). In the past 12 months they've gone from 100,000+ paid seats to over 800,000+ paid seats. The investors are valuing (and expecting) future growth.

For comparison: Atlassian is estimating $440+ revenue for their fiscal year, and has a market cap of $5.5B.