I personally think you are getting SPAM from SEO oriented directories. Have you promoted your website at some of the smaller or niche search engines or directories? Many Optimizers set these up to get your info and make an offer. There isn't anything wrong with that, but you need to be aware that as you tell people about your business, non-ethical people and businesses will have it, too.
There is also a scam that is generated by having a listing in Google, people call to warn you that your Google My Business Listing is expiring or needs help. But, they don't represent Google at all. They are optimizers trying to make a buck. And quite frankly, there are many less tricks to increase your Google Rank, your Domain Authority, your Page Rank or Alexa Ranking other than simple common sense approaches.
That said, Google is now getting pretty picky about how a website is designed. There are now warnings about using a font that is too small to read, that link items are too close together (because of touch interfaces on tiny phones and tablets that are not real precise).
But most of that is also common sense. They are getting pickier, though, and starting to act as a dictator about how your website should be designed, which I have a real problem with, they are directing designers, and that is not their duty. Google is simply becoming way too dictorial. It gets away with it because it IS a monopoly that has existed for a very, very long time.
It is also common sense to have an https:// enabling encryption certificate. There are 2 types of certs, one is paid, where you have to verify who you are, and another is free. Apache and LetsEncrypt and others have free TLS/SSL certificates, and these have to be renewed every 3 months. Many, many web hosting services offer a free "SSL" (they usually use TLS encryption, now) security certificate that will automatically renew which you can setup on a free shared IP right from your web hosting control panel (even I offer the LetsEncrypt in the CP and it renews every 3 months as long as the domain hasn't expired).
The TLS cert allows you to pick-up your email from the server securely, and allows any https:// transaction to be encrypted so that privacy is maintained between the client (browser) and the web server. You do get a big hit when your website doesn't have one implemented. Especially if you allow your visitors to submit any info via a web form, including a newsletter sign-up, because email addresses are private info, too. In this case, its about treating your visitors privacy with respect, which is also common sense.
But everyone else has hit on the most important thing about Domain Authority, you need original content that is vital to the subject or shows/explains/teaches others (humans) how/what/why about a subject that you enjoy and are passionate about. Write it well. Decide who your audience is. If you are targeting pros, you won't have to do so much step-by-step details. If you are targeting starters, you will need to walk them through everything, including terminology. And that terminology can be effective as keywords, too.
However, there is a place for Optimizers (aka SEO Pros), if they can do their job effectively. And that is helping the guy/gal starting out to be a real force when they don't yet get the web. A good SEOer will explain the theory behind everything in basic terms that a webmaster, admin or owner can relate to. It's kind of a part of the job, so you know where your money is going. So they do have a place. But, to have results within 15 days is one of those promises that sounds unreasonably short. And such promises as this, always incredible, seem to speak to whether or not a pro is ethical, and your website may suffer in the long term.
So, be very careful with such ideal promises. I would've deleted that email based on the fact that it was too spammy. If it is (a) friend(s), maybe you can investigate further, but I wouldn't. Don't get suckered into promises that can't be kept ethically.
You do a good job with your website though, and you will do well after a month to 6 weeks, but never sit on your butt, either, keep it updated with new and relevant stuff.