I think if you consider the increase in assets (total value of the # of servers, racks, network gear etc) from the purchase of hardware to accommodate that growth, in order to satisfy those revenues and future revenues, it makes sense. It is entirely intentional and desirable for any gross margins to go fully back into growth or assets to support growth - especially in the cloud game where infrastructure costs and R&D are high and front-loaded. Once you purchase a hard drive and place it in a network-connected server, it will cost a few pennies each month to keep it spinning while it will generate revenue for 5 years (or more) after. You can see this by looking at reported Total Assets increasing from $38M in 2019 to $54M in 2020. That is after depreciation mark-down of <2020 equipment which is typically very high for IT equipment (upwards of 33% per year of the original value). The increase in annual revenue over time from previous cohorts is very attractive and shows that customers do appreciate the cost effectiveness and quality of service compared to other offerings.
If you consider they bought & added, net of depreciation, about $16M of assets to their balance sheet in 2020, the loss of 6M doesn't seem terrible. Using the bean counter approach - no new marketing, no spending on growth, no unnecessary R&D - they could have cleared about $20-25M of ebit in 2020.
Deciding to use your gross profit to fuel further growth (especially considering they're compounding it at a healthy rate and doing so without giving up equity or interest) is a wise move. If anything, it probably would have made sense to take some debt or finance more of the hardware purchases early on (as I see they are doing now at some scale as of 2020) in order to put those proceeds towards customer growth.
The only thing I see which is a marginal concern with these numbers is that it's difficult to see the split between marketing spend on b2 customers and backup customers (They spent $8M in 2019 on sales and marketing to generate 42k new customers, and $11.9M in 2020 to add 40k). It is very possible and likely that those marketing dollars went towards new enterprise customers (on b2 storage vs backup) and the revenue-per-customer number increasing during that timeframe seems to support that. Are blog posts calculated in the sales/marketing spend? Seems lots of the customers came from the organic content play.
Very cool to see the company who's free blog post about which harddrives are most reliable (when some had >5% annual failure rate and others <1% for the same price) nearly 10 years ago come around to s1.