Ask HN: Is SEO worth to buy from an ad agency?

While trying to buy some time to build my portfolio, I've got a (probably temporary) job as a bureau coordinator for a small translation agency. They want to grow their client base and want me to create a website for them. I am already managing their social media and Google business profile and as I had a litle experience in building a Wordpress site I plan to use "drag and drop" website creation tools provided by hostinger (Wordpress or developed by hosting provider), which include SEO (internal).

The competition in our city is pretty high, though much less in smaller towns. I generally have no experience in SEO, just know the basics and watched a bunch of random webinars and videos on the topic (also purchased some cheap Wordpress plugin to rank on google and never used it).

Recently I've got a call from a local ad agency, offering to create a full-pledged website with SEO, security etc and ensuring it to rank on the 1st page of Google search results in 2-3 weeks upon launch. They provided examples of websites ranking on google they created for other local businesses and generally have good rating. They say they also ensure backlinks for external SEO.

My agency is already using a local marketing provider with a monthly billing, which distributes the information across its publishers, which seem to help a lot.

Should my small agency buy into this offer? Apart from SEO, they also offer to create cookie policy and ensure security, which also were not part of my focus.

There's one thing that seems strange with SEO (at least on an abstract level). This ad agency puts a footnote at the bottom of every site they created with the link to their site, telling who has created the website. So I may suppose, if they create an SEO ranking website and our competitors will see it ranking on the first page of Google, they will see this footnote and may also want this ad agency to create such ranking website (which may be unlikely, but still a possibility). So when 10 websites operating in the same niche will rank on Google with the help of this ad agency, then there will be no place for my small agency to rank on the first page (or maybe they will rank for different keywords?).