Although you are absolutely correct, and indeed the 26-30 railcard states on it to claim the 16-25 discount if the 26-30 does not appear, they do have slightly different conditions of use. A few people I know have been penalty fared for using 16-25 rather than 26-30 for the discount, reason being 26-30 has the minimum fare before 10am in July and August and the 16-25 doesn't. As always, pays to read the terms and conditions I guess!
My 26-30 used to say "Valid with 16-25 and 26-30 tickets" in the app, but it doesn't any more.
I accidentally did this last week, fat fingers pressed 16-25 rather than 26-30 on the ticket machine, but I thought it would be fine as I remembered seeing that wording in the app.
As it turns out I had to swap a journey before 10am with one after 10am at the last minute due to a delay, but I wasn't thinking about that as I had 20 minutes to run between the Wakefield stations without knowing the way (having never been there before).
When I got on the train I realised my discounted ticket which I had intended to use after 10am was not valid at this time, then I checked my railcard app and it no longer says valid with 16-25 tickets, so I was prepared to have to buy a new ticket, but the guard didn't ask for railcards or even read the stations on my ticket, so I "got away" with it.