It's probably just like your art contest picture shows so well, he doesn't have great capacity to link things together yet.
Do budgies love millet spray just like tiels do? If they do, you can't fail with that. Unless you are one of those 1% unfortunate bird keepers whose bird doesn't like to gorge on it!
When I watched the treat-training videos for the first time I thought "Yeah right... It can't be so straightforward". Then I found out that it is. They NEVER forget that
They remember the oddest things, IE. colour-coded food bowls even if you don't use them for months, so I can't believe that Poppy can forget the training just like that.