Let me try and clarify a few things.
AdMob, inMobi, Vungle, Chartboost, etc. are "ad providers". Advertisers buy ad campaigns with them to advertise on your app. This is who you talked to. They thought you were an advertiser wanting people to learn about your product.
Appodeal is an "ad mediator". They take all of the ad providers, find the best ads for your app and show them. AdMob is one of the ad providers that Appodeal can use as part of their offerings, but if you use the modular (recommended) version of their plugin, you do not have to include AdMob at all.
Appodeal is free to use and you can use AdMob for free through them. If you want to use AdMob without using Appodeal (or Chartboost, Unity Ads, or AdColony), those plugins require a fee to use. Other providers like Vungle, inMobi and others are totally free for you to use.
Many people will start with a single ad provider to get used to working with ads. You probably should start here. Appodeal really needs a fairly popular, high usage app to outperform individual ad providers.
There are a couple of concepts about ads you need to understand too.
First, different ad providers have different fees they pay you. Typically measured by eCPM (effective Cost Per Mill or per 1000 ads delivered) one provider might have an eCPM for interstital ads of $1.00 / 1000 while another might have an eCPM of $5 / 1000. So why not just use the one that's $5? Well...
Secondly, different ad providers have different fill rates. Your app might ask to show an ad and that provider doesn't have one for you. Because of this your $5 eCPM advertiser make take 5,000 attempts to show 1,000 ads meaning at the end of the day, you still only made $1 for the attempt to show those ads.
To get around the "fill" problem, many developers will setup a waterfall system where they try their best eCPM provider first, if they don't get an ad, they then attempt to show an ad from their next best provider, if that fails, then they go to the next providers either eventually giving up or using a high fill, low paying ad service.
This is kind of what Appodeal does for you. It finds you the highest paying ad and gets works to make sure you always have an ad. But for mediation to work, you need like 10,000 active users a day for the mediation to work right. Advertisers don't want to advertise on apps that no one is looking at.
AdMob, at the end of the day, has the best fill rates around the world. You might find that another provider will have a good fill rate in the US, but a terrible one in France. AdMob also has one of the higher eCPM rates you can get making it the most attractive ad provider. You, of course, could build your own AdMob support into your Corona app if you do native builds, but if you put a value on your time you will find it's actually cheaper to use our plugin rather than you having to learn native iOS and Android development and work with native builds instead of simulator builds.
My recommendation is for you to focus on building the best game you can. Think and learn about monetization types (banners, interstitial ads, rewarded video) and see how your app can work with those types and perhaps start with a couple of the free ad provider plugins. Spend time getting analytics in your game so you can get an idea of how many daily active users (DAU) you have, how long your sessions are and other metrics that can help you figure out the best way to monetize the apps, then you can figure out if there are good places to show full-screen interstitial, interruptive ads vs. placing banners or if you can give value to your users allowing them to choose to watch a video. You can start with Appodeal without AdMob if you wish.
If your usage shows that AdMob would be a better choice, buy the plugin, setup the AdMob account and then you will have the activity there to make Google happy. If your usage gets to the 10,000 DAU level, then look to Appodeal (with AdMob setup).