And that's not even the totality of their potential customer base. It just happens to be their primary focus (and pricing model) right now.
There are a huge number of repetitive processes/workflows that simply don't bubble up to the purview of IT. They either don't directly intersect the systems centrally managed by IT, don't warrant the cost/time/effort required for IT to get involved, aren't formally documented or known by IT in the first place, or IT simply doesn't provide any official channels to engage with them for automation-related support.
UIPath's current market positioning and pricing scheme is geared solidly at business cases built off of "ROI of RPA vs. IT modernization efforts". With a focus on automating singular processes occurring at a high frequency. Which alone is enough to support their valuation. But they could also trivially expand their positioning to include "power user" oriented pricing plans and move into that area of the market as well, where singular users (or teams) have a high quantity of low volume workflows.