Unfortunately, it's not possible with smart collections. You could use filters instead of smart collections, however. In the Keyword List panel on the right, hover over the "Remco Douma" keyword and click the arrow to the right of it:
That will filter the current photos to show just the ones containing that keyword.
Many people have complained about the lack of exact match for keywords and other metadata fields in smart collections -- it's a big design goof by LR. Please add your vote and opinion to this topic in the official Adobe feedback forum: Lightroom: Exact text match in smart collections and filters, including matching spaces | Photoshop Family Customer Comm…
Library filters can search many of the same metadata fields as smart collections and you can save filter criteria as presets. Though they can't search all the fields that smart collections can, they have a couple of big advantages: they can do exact match on keywords, and they can be applied to individual folders and collections. (Smart collections can't easily be applied to individual folders and collections, another design goof.) But if you want more power than filters provide, check out my Any Filter plugin -- it searches many more fields than smart collections, and it has many more matching operators, including exact match.
Tip: In general, it's better to use "contains words" rather than "contains all", since you're less likely to get false matches.
Thank you John. Coming from Aperture, I'm flabbergasted that this isn't supported by default. I voted, but given that that post is 5 years old it seems like Adobe couldn't care less. Quite ridiculous, really.