You're completely right, I had forgotten about constructive geometry. Thanks for pointing it out!

I think you're conflating two things: formulas and numbering systems. Ancient Greeks still operated in decimal albeit they'd use the Greek alphabet (since dedicated glyphs for numbers didn't exist) but those letters followed a base-10 system. They would have done the same finger counting we did as kids. The difference between then and now is their calculations were geometric instead than algebraic, rather than them not thinking in decimal.

In that regard your argument is a little like saying kids test question such as "Jennifer has 3 apples and gives one to Bob and Terry, how many does she have left" isn't decimal because the calculation is written in English rather than as a "number sentence" (as schools call them). But clearly the kids are still thinking about those numbers as decimal even if the question is not phrased as a mathematical equation.

Whereas some other cultures used base-12 or base-60 (https://en.wikipedia.org/wiki/Sexagesimal) which, incidentally, are also believed to originate from finger counting much like base-10 was.

I believe the Romans also thought about things in decimal despite their numerals being logarithmic. However their numerals do follow the decimal orders of magnitude.