The 'good' developers will attend as long as there is no hidden small print that says any idea's or creations created at the workshop becomes the property of Apple and thus they can use it as they wish. It is no different to any business or online entity. You get hired by a company, you look through their employee's hand book and it says 'anything created during works time because the automatic property of the employer'. Facebook, instagram, tik tok, reddit, whatsapp and many many more, all have small print that say's anything posted on their platform they take ownership and can use as they wish.
Developers are not going to smash out thoughts, suggestions and idea's in the presence of Apple if they know Apple are going to take these thoughts, suggestions and idea's and use them as their own, even if the developers do nothing with them.
This why if your a designer of something and get invited to an event you need to read the fine print of the invitation and the terms and conditions of the event because I bet you there will be wording that says everything going on in the event the event holder takes ownership and can be used by them.