We currently don't expose exact numbers used in these sort of calculations, sorry! (plus formatting is inconsistent between tooltip and Describe Trend Line dialog)
Curious though - why do you have a need to replicate this formula?
I am working with cross-sectional data, so I was trying to see how someone's probability of Y occurring changes, given their value for X. So I fit a trend line using all the observations that I have both X and Y values for and was hoping to apply that trend line to those observations that I only have X values for, to get a predicted Y value. However, as you can see in the packaged workbook, the predicted values aren't coming out quite right.
I ended up just using Excel's LINEST array formula to obtain exact coefficient values.
You're right it's a significant difference on a graph and its not a simple transformation. I'm not the most skilled R programmer in the world in fact probably barely even considered an R anything. But this might be a case to link Tableau to R and to pull the calculated trend line data off from it, especially something as large and complicated as a 5th degree polynomial trendline to calculate. Have you looked into this route?