I agree that your explanations are quite possible and hinted by the reported facts. But also, there can be communication problems inside the startup as well. Maybe the people sending the strongest "can't wait for your return as full-timer" signals hadn't yet heard others' concerns. Maybe even the founder and lead engineer are being squeezed from other stakeholders. And even though a legal complaint is unlikely (and unlikely to succeed), liability concerns can lead to strange omissions and spin in the 'official reasons', to provide maximum legal cover 'just in case'.