It's a good question.
I believe one of the criterium for lifetime platinum is 12 years. I'm not sure if it's 12 years in the program, or 12 years from first achieving platinum, but there have been lots of posts here on Insiders referencing the criteria. Do a search on lifetime platinum in the search box for more info. If I or anyone find the link to the Marriott webpage that spells out the criteria, we'll post it.
Here's one post:
Also, look to the right of the screen for the header in orange 'More like this.' Those threads might help.
Here are more links that may be helpful.
I believe vickisocial is exactly correct.
As noted above, they probably are Lifetime Platinum (they certainly should be). But unlike some other programs, there is no welcome/ceremony/etc when you hit that level. You only find out when you fail to meet PLT criteria one year but are kept at the level with your lifetime status. There have been mixed stories on some people getting welcome letters while at the status, some MR cards with an expiration date of "never", etc. But I've reached lifetime silver and gold levels, closing in on platinum, and I'm never received any indication.
MR has announced that they are changing/modifying/formalizing the "undocumented" lifetime status for the upcoming year, so I suspect in the future it will be more obvious, such as SPG's "Lifetime ***" notation on the member cards.