Not sure if it is a compulsory part of the spec, and irrelevant for most laptops since laptops generally wouldn’t use DDR5, but use low-power DDR instead.
On-die ECC should help with data integrity, but on-die ECC doesn't protect integrity all the way to the memory controller, and lack of reporting means I don't think you'll even know when there's an uncorrectable error. Which means you've still got the same basic issue --- memory is not stable, although the error rate is likely reduced.
Not sure if it is a compulsory part of the spec, and irrelevant for most laptops since laptops generally wouldn’t use DDR5, but use low-power DDR instead.