Just got off a support session with a (very confused) Google Support guy and the gist of it is that it is still not possible to trigger Google Photos casting from your phone to the Chromecast via voice commands. It works on the Home devices but not phones.

Here is the chat log if anyone is interested:

Chuck 3:21 PM Thanks for contacting us! My name is Chuck. I'm happy to answer any questions you have about Google Home.

Chuck 3:21 PM Hi!

Chuck 3:21 PM Hi, am I chatting with X?

Me 3:21 PM hey there

Me 3:22 PM yes you are

Chuck 3:22 PM Hi X. How's your day doing?

Me 3:22 PM so I am trying to cast my google photos to my tv using my pixel xl and a chromecast that is hooked up to my tv

Me 3:22 PM but the assistant only shows google results or google photos results and never starts casting them

Chuck 3:23 PM I understand. Let me check that right away. May I know which country you are located in?

Me 3:23 PM Germany

Chuck 3:24 PM Give me 1-2 minutes while I check on this.

Chuck 3:26 PM I also see that you have visited our support site for View Google Photos using Google Home. Am I right?

Me 3:26 PM yes

Chuck 3:31 PM I'm looking at the information, what you need to do is activate "Personal results" of your Google Home.

Me 3:32 PM that is already turned on

Me 3:32 PM also I was trying the voice command on my phone, not on my home

Chuck 3:32 PM By the way, are you getting an error whenever you cast Google Photos

Me 3:33 PM no, if I cast them manually it works fine. Just when I want to trigger the casting via voice commands it fails

Chuck 3:33 PM Are you using Google Home device to cast the command?

Me 3:33 PM like I said, I use my phone

Chuck 3:34 PM Do you have a Google Home device?

Me 3:34 PM yes

Chuck 3:35 PM Does it work on your Google Home device?

Me 3:36 PM Yes it does

Chuck 3:38 PM I'm still checking the settings of the Google Assistant. Will you give me 1-2 more minutes.

Me 3:38 PM Sure

Chuck 3:38 PM Thanks X.

Chuck 3:39 PM Are you using the same account with your Google Home app and Google Assistant?

Me 3:39 PM yes

Chuck 3:41 PM Are you using an Android device or iOS?

Me 3:41 PM Android, a Google Pixel XL

Chuck 3:43 PM I got it. Can you open your Google Assistant and tap on the blue button on top.

Me 3:43 PM yes

Chuck 3:45 PM After that, tap the three circles on the right, it should show a menu. Tap settings, then go down and check if you see "Videos and photos".

Me 3:45 PM the google photos toggle there is already enabled

Me 3:46 PM I already tried turning it off and on again but it did not fix it

Chuck 3:48 PM I see. I have double check the information here on our end, and it seams that this feature for Google Assistant is still not available.

Me 3:49 PM So the feature does not work on phones but does work on the Home?

Chuck 3:49 PM Although, this feature is available for Google Home its not yet available in Google Assistant.

Me 3:50 PM ok, thank you

Chuck 3:50 PM Is there anything else I can help you with today?

Me 3:50 PM No that's it, thank you :)

Chuck 3:50 PM Thanks for contacting the Google Home Support Team! It was a pleasure chatting with you. Have a great day!

Chuck ended the chat

tl;dr Even Google is confused about their products and staged rollouts and features and their support staff can even see which support articles you've already read.