They walk away from their computer for a week and come back, expecting the shopping basket to still be there.
I wonder what's wrong with these people? Or are they getting this "feature" from other sites such as Amazon, which sneakily makes sessions in the back and accounts for people as they use the site? (which might be a better way of dealing with this... instead of demanding they do some kind of work, just wait til they're already doing some work anyway and re-use it to improve user experience instead)
So this is supposed to be helpful and useful, but is actually Clippy-annoying and possibly inaccessible... so here's some better ideas, I think. (if you use these ideas, you don't have to worry at all about SEO)
You could instead offer a one-short-sentence "alert" ONLY when someone new-and-not-logged-in adds their first product to their basket, saying "see how your email address can remember items in your basket for you" (or similar, and preferably shorter) and a plain old link to that longer text you posted above, as another page or something. Since popups aren't fitting on mobile screens, either only popup info for desktop or just let everyone go to a separate page. People don't mind separate pages, really.
I like this a lot: it's rather unobtrusive, and it only appears when the user does an action where now your email-scheme would actually benefit them. And they still have the passive option of just ignoring it, rather than needing to click Cancel.
The alert should either show up wherever their basket stuff is (if it's the usual top-right area most places put these things), or better yet, near the button they clicked to add the item to their basket. If an automatic page refresh happens at that point instead, then wherever user studies show people's eyes go to first thing on that page. You'll need to test this to see if people miss it entirely, because if nobody sees it, it will never work, and whomever suggested the ginormous annoying popup will have had a "better idea". So test whatever you implement on a few random (not terribly familiar with your site) people.
I think that's probably what you want: OFFERS information WHEN it benefits people WITHOUT requiring any action. People who get all pissed that their unsaved baskets are empty when they come back from Hawaiian vacation are the people who will care enough to click that link. Everyone else will shrug and say "well yeah, nothing else gets remembered when I close a page or sit too long either". Or maybe I'm weird, but I never expect things to be remembered. Especially e-commerce, where in the meantime that thingie in my basket may have sold out now, or changed price.
If you're worried specifically about screen readers, maybe maybe maybe an aria-role="alert" or aria-live="polite" might make sense (live regions tend to be things sitting on the page who change regularly from user interaction, so probably alert makes more sense). I use it for when users add stuff to their baskets and the little basket area in the upper-right of the page changes the number of items and price. It unobtrusively lets users know their basket action worked, updates them with very short information, and doesn't disturb their keyboard or screen reader focus.
In addition, you can and probably should repeat this information as a single sentence + link-to-longer-text, just regular in the page (no popup or JS, and don't show or display:none to people already logged in), at the top of their View Basket page, or near the View Basket button/link, and perhaps when they are already filling in their email with all that other information they do when they actually go to buy (remind them that later on they can use their email address to save baskets and wishlists, again with a very short one-sentence bit).
You could, on those pages, even style it as a "handy tip" with a little related icon. It then stops being a pain in the a$$ for customers and browsers (by browsers, I mean people who look but don't intend (yet) to buy), and becomes something helpful and useful.