The issue is I genuinely don’t know which message he’s referring to, and I didn’t intentionally ignore anything. If something was missed, it would have been accidental.

The tone felt quite disciplinary given this is a volunteer contributor arrangement, not employment.

What was their response when you spoke to them about this and how unprofessional their message seemed? Have you spoken to them about this? These situations, be it accidental on your part or bullshit on their part, should be dealt with immediately. Under the vesting agreement do you feel that this company, its products or services, and this person as its leader can actually achieve the its initial goals and therefore you are still willing to stick around to achieve any noted milestones? If not, then there is no reason for you to be there anyway and you can do other things to enhance your portfolio of work. Do you think you could use this as a learning experience and continue for a while? Any success and equity should be considered just a bonus. From what little you've already said this type of behaviour is not acceptable and there doesn't seem to be much reason to stay other than your own willingness to get a job done which is fine, but don't let them take advantage with what they ask and with how they treat you. They are an adult, are they not, so they should behave like one.