swamp vs fill up

swamp is a noun but fill up is not a noun.

Word	Noun	Adjective	Verb	Adverb
swamp	Yes	No	Yes	No
fill up	No	No	Yes	No

As verbs, fill up is a hypernym of swamp; that is, fill up is a word with a broader meaning than swamp:

Other hypernyms of swamp include fill, make full.

swamp (noun)	fill up (noun)
low land that is seasonally flooded; has more woody plants than a marsh and better drainage than a bog
a situation fraught with difficulties and imponderables

swamp (verb)	fill up (verb)
fill quickly beyond capacity; as with a liquid	make full, also in a metaphorical sense
drench or submerge or be drenched or submerged	become full
	eat until one is sated
	fill or stop up