1. Cellphones are a kind of personal computer.
2. Numerical computation is something that computers, personal or otherwise, are very good at. Conservatively, your cellphone is ten orders of magnitude faster (ten billion times faster) than you are at tasks like averaging a set of numbers.
3. The spreadsheet user interface is expressive enough for many numerical computations† that are impractical to carry out with more limited user interfaces such as pocket calculators, but it is simple enough to understand that large masses of people can take advantage of that expressivity. (The popularity of VisiCalc on early personal computers such as the Apple ][ is one piece of evidence for this.) It is the "low-code development platform" that inspired all the current no-code and low-code platforms.
4. Such numerical computations are so commonplace in many people's lives that they do them on their cellphones, despite the small display and lack of a keyboard; one reason is that many people have cellphones as their only programmable computers. When they do such complex numerical calculations on their cellphones, they often use spreadsheets to do them.
5. Therefore, we should regard the availability of spreadsheets as a central indicator for the viability of a computer software ecosystem, even on cellphones.
I think all of these claims are obviously correct, stipulating the ones before them, except for #4. As evidence for #4, https://www.youtube.com/watch?v=RCpJ441g-Y4 shows that the Google Sheets app for Android was at the time #7 in their "productivity" category with 793000 ratings and 4.8 stars. https://play.google.com/store/apps/details?id=com.google.and... says that it has been downloaded more than a billion times and has 1.27 million ratings. The fact that people exist who do not use their cellphones for spreadsheets does not constitute evidence against this claim.
What I believe is happening, to elaborate a bit more, is that F-Droid users who need numerical computation that goes beyond what calculator apps can do are mostly just using the Google Sheets app. The radical fringe of F-Droid users like me who do not have Google accounts often make do with Termux programs such as Python, LuaJIT, PARI/GP, bc, Racket, or the C compiler, even though for many purposes a spreadsheet would be much more convenient.
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† Spreadsheets are also used as simple databases, in fact more frequently than they are used for numerical calculations, but numerical calculations alone are a strong enough argument for my purposes here, and F-Droid does have a number of adequate simple database apps.