It was intended to mark a site the adventurers might visit
The map indicates that X marks a "Site", and the adventure states on page 31 under the heading Valley Sites:
The Dessarin Valley has been a well-used highway into the heart of the Sword Coast North for an age upon an age, and it has seen the ravages of many orc hordes, the rise and fall of a dwarven kingdom, frequent bands of brigands, and much more. Adventurers exploring the Sumber Hills and vicinity might visit many sites of interest as they trek across the valley.
This then is followed by various sites and cities that can be visited. However, nearly all of the sites marked with X are described in more detail in the adventure in case the characters want to explore them.
The exceptions are the "Wyvern Tor" and the "Fire Raiders" sites: both do not have an explicit entry or description. I think this shows this must be an editorial oversight or mismatch between the map and the adventure text, as "Fire Raiders" certainly is not an enduring landmark that one would want to visit. These "X"es represent sites that were at some point planned to be encounter/exploration areas for the adventure, but some of them were not included in the final text.1
1 There are mentions of fire raiders on the random encounter tables but without a fixed location, and there is a "Rescue your People" adventure hook where raiders from the fire cult have abducted people, but those are in the Temple of Eternal Flame, not in the open plains near Kryptgarden Forest, so they are not that Site.
P.S. If you compare the maps of the Dessarin Valley with the one in Lost Mine of Phandelver or Dragon of Icespire Peak, those show Icespire Peak, about 5 miles east of Conyberry. This would still be on the Dessarin Valley map, and much more of a landmark than Wyvern Tor, so it should warrant an X if an X was to highlight landmarks, but it is not included.