Yeah I think it's weird they highlight that distinction since you can only contribute for under 18 folks anyway. How many under 18 employees wanted to contribute to their own Trump account? Pretty much a non-issue.
The BIG deal I think from this is that it seems to suggest employees will be able to make pre-tax salary reduction contribution elections (presumably up to $2,500, reduced by any employer contribution) for Trump accounts of a dependent. There's no way to make deductible contributions outside of payroll. So all of a sudden the name of the game in Trump accounts is going to be to get your employer to throw them into the cafeteria plan, and then always make sure to utilize the pre-tax option through payroll before ever considering a regular nondeductible contribution.
Given that most employers are working with a FSA TPA that offers a variety of cafeteria plan benefits in a unified login (health FSA, dependent care FSA, commuter, HSA), it seems that adding Trump accounts with employee pre-tax contributions would be an easy flip to switch to offer a pretty meaningful benefit to employees at almost no cost.