"Citizens United declared it was unconstitutional to prohibit that corporations and unions spend from their general treasuries to finance independent expenditures related to campaigns, but did not alter the prohibition on direct corporate or union contributions to federal campaigns."
https://en.wikipedia.org/wiki/Political_action_committee
It is Mike Thompson's position that he has not accepted 50 donations from PG&E, but 50 donations from a PAC controlled by CEO Andy Vesey (or previous CEO Geisha Williams) and the PG&E Board of Directors.
Citizens United says it's the same thing and so do I.
If what he is saying were true, Democratic Presidential contenders would have eaten Senator Sanders for lunch long ago.