This is an ad for a proofreading service, so nominally it’s meant for you to use in formal writing. Moreover, only a small proportion of these words are “fancy”.

That said, a thesaurus is best used for remembering words you already know, i.e. not this. Careful use of a thesaurus to find new words provided you research them first – e.g. look them up on Wiktionary (bang !wt on DuckDuckGo) to see example sentences, etymologies, pronunciations, possible other meanings, usage context (e.g. if it’s field-specific jargon), whether it’s appropriate (e.g. slang, archaic), etc. – can work, but if you’re already writing something, just stick to what you know unless it’s dire. You should make an effort to learn words over time as they come up in appropriate contexts rather than memorizing them replacements for other words; this infographic offers a shortcut that’s probably harder and less accurate than actually learning.

A one-night stand with a word you found in the thesaurus is going to alienate people who don’t know what it means and probably make you look like a jackass to those who do.