I don't have time for the next few days to test it, but I am pretty sure I did search from within the PM, on mobile. But I'd have to look again to be sure.
When it didn't work for me on mobile I then switched to desktop, which did not work either because the search within topic was not selected, even from within the message.
So I suspect this is going to come up again - people are going to get stuck on this step if having it selected depends on being right in the message and doing the whole thing live, without taking a break from it.
ps - another related issue is that on my site the discobot messages are being delivered by email as well, so people are going to try to complete it later when they see their email and click through to discourse.