That is way too simplistic (and wrong).  I am not going to argue the moral/ethical issue, just the legal point.

1) First, nothing would be "taken" from the possession of GAWCEO.  Conversion (which is the legal cause of action for theft), whether civil or criminal, requires that the wrongdoer take from the rightful owner possession of the property.  The loss of a possessory right is central to a claim of "theft"/conversion.  A fork in the blockchain would certainly interfere with GAWCEO's ability to use that which he possesses, but would not deprive him of possession of anything.  Comparing GAWCEO's possession of XPY and wallets a microsecond before and after the blockchain fork, there is no loss of any possession. 

2) You are really making an expectations argument (estoppel) or an implied contract argument (breach) I think, but that is not a theft or conversion issue.  And I think any claim by Josh on an estoppel theory would fail (he cannot assert an equitable argument because he has "unclean hands"), and an implied contract claim would fail for the reasons previously stated (new contract, new consensus does not breach old contract).

Consider this:  If instead of hard forking, the same folks get together and create a new coin called Paycoin Redux (PAX).  They do an ICO in which any holder of XPY can obtain equivalent value pro rata percentage of PAX coin by exhanging their XPY for PAX; except Josh and any person or entity that can be identified with Josh as an affiliate or family member is banned from ICO participation.  The XPY exchanged is destroyed.  The result of this would be exactly the same as a hard fork, but you cannot possibly claim anything morally, ethically or legally wrong with that.