Yep, definitely a big omission. The page notes "Not shown here, Americans also each eat about 16 pounds of seafood, with shrimp the biggest component at over 4 pounds. This represents on the order of several ten billion fish and shellfish."

There are probably a few relevant factors in their decision to omit all sea animals:

• Perhaps the data isn't as good or as clear for number of sea animals. Sometimes it may be measured by weight, and then there are the problems of bycatch and sea animal bodies that are fed to farmed fish.
• Historically, traditionally, sometimes the word "meat" has referred to mammal (and bird) flesh. Maybe this is because fish are and other sea animals are seen as quite different than mammals (and birds).