Hi Jack,

I can't find this example in the lesson but I am assuming they would say -

La dinde était tellement brûlée qu'il avait dû aller en acheter une autre

The pluperfect is correct, don't assume that every time you have a second clause introduced by 'que' that the subjunctive will follow.

Take a look at the following Kwiziq lesson which will give you some sub-lessons when the subjunctive has to be used -

https://progress.lawlessfrench.com/revision/glossary/mood/the-french-subjunctive-mood-le-mode-subjonctif-ou-le-subjonctif

Hope this helps!