That depends entirely on how you raise the funds. Yes, you can say "Here's the growth rate we'd get without your money - based on that, this investment gets you an ROI of x%."
With x% high enough, sure, you can get VC money without too many strings. (Also, reading the Series B post, they were planning to invest - just in organic growth instead of the usual growth hacking)
And if you read the Series C post, you'd know what they're spending on - GPU (and general) cloud interconnectivity.
There's really not much need to guess, Tailscale's financing announcements are about as open as you can get.