Additional suggestion: many times the home page is a link to many different technologies. Crawl all first-level directory indices to see different techs. E.g. we have a xenforo-powered forum at /forums, a WordPress blog at /blog, a custom ASP.NET CMS at /store, a .NET Core web app at /foo, etc.
The domain index for most companies past a certain age/size not dedicated solely to a single app effectively turns into a static html page.
It does have the somewhat negative effect of making potentially vulnerable websites more visible to lower order hackers (I'm assuming more proficient ones have automated discovery tools like this anyway).
I learned a few years ago from some DEFCON video that someone had figured out a way to do a (basic) port scan of the whole internet in ~1 day (or something like that).
Thing is... it really shouldn't have been that surprising. Although network latency isn't getting that much better year by year (c = c), the amount of data you can process in bulk, correlate, etc. is ever-increasing.
 At least, I think that was the conference.
"This program spews out packets very fast. On Windows, or from VMs, it can do 300,000 packets/second. On Linux (no virtualization) it'll do 1.6 million packets-per-second. That's fast enough to melt most networks."
Massscanning the Internet - Defcon 22 (2014)
(I should add that I forgot to mention that IpV6 does make the whole "PortScan the Internet" business a tad more complicated, so that's than argument against me.)
Submitting your site to this is just asking for trouble.
My web logs are full of automated scanners. Once when I ran a vulnerable version of Wordpress it got discovered and pwned very quickly. No need to enter the URL in any website ;)
It's kind of clever marketing, giving people a sense that they're going to get a security audit in exchange for an email address.
I hope that if you subscribe, the site regularly rescans your stack and realised if it’s changed. Otherwise it’s just a mailing list subscription that becomes out of date and therefore not useful.