A caveat that I considered adding to that statistic but left out due to brevity:
30% is likely over-estimating because there is likely some # of people that still work on Heroku but labeled themselves as Salesforce employees. There are also probably a higher number of people that work on Heroku part-time so also label themselves as Salesforce.
A tool like your suggestion could probably help but it would include folks that transitioned to a different part of Salesforce entirely.