I do front-end dev for a high-volume recipe site and our multivariate tests mostly confirm the opposite. Simpler pages rank higher (and users report higher satisfaction with the product). Core Web Vitals changed the way a lot of things work, how long ago were you given this advice?
A while back (two years?). The thing is, user were extremely satisfied by the product. The users came to the website for a comparison table (which was at the top), used the information, clicked a link on the table and then exited the website. The rest of the page was useless, most won't even scroll. But still they needed it for a ton of SEO reasons (keyword density, semantic structure and complexity, internal and external linking).
The company was working on an extremely competitive niche and it was crushing it (multi-million dollar ad revenue), so I think they knew what were doing.