Will I be able to test port forwarding to an iPhone behind a firewall
Will I be able to test port forwarding to an iPhone behind a firewall

There's no reason why you shouldn't.  Technically, an iPhone is just another node on the network.

It appears that the tool will not be able to reach an iPhone behind a firewall. Is this assumption true?

It depends on how you configure the firewall.  The screen shot you posted &quot;says&quot; you've forward a number of ports, some of those related to Facetime.  For the record, you need to forward ports 53, 80, 443, 4080, 5223, and 16393-16472  (UDP).
The question is, where did you forward the ports to?  If nothing responds, whatever tool you use will assume the ports are not open.  Think of it as calling a phone number.  Whether or not the number is blocked, if the person at the other end doesn't pick up, do you know if they are home?
There's nothing in that screen shot that tells us how the port forwarding is configured.  It's just a list of ports at this point.

The person on the other side is using a tool like &quot;what is my ip&quot; to see their IP and I am using that, plus a port on that list to test.

Congratulations.  You just added your IP to a database of IPs that have some sort of web service running on it - in other words, an attack point.   If you're going to do a port scan, use a tool that runs locally on your machine and not on some unknown server.  I have seen many instances where unsuspecting system admins would utilize these sites to test their firewall only to be on the receiving end of endless connection attempts from around the world including Russia and China.
Could they have found eventually? Yes. But running a port scan, especially when it responds on well known ports like 80, 8080, 443, 22, etc, you're confirming there's a server there.
Bottom Line
This is actually a networking question and specifically how to configure a firewall and to test those ports.  The end device being an iPhone is really not germane here as this issue would exist with an Android or a PC. Your router manufacturer should have HOWTO guides on firewalls and port forwarding.  From there, take a look at https://networkengineering.stackexchange.com/ here on Stack Exchange for more in depth information regarding firewalls and port forwarding.