Learn German: Ich finde die Diät insofern gut, als sie hilfreich ist. - I find the diet good insofar as it is helpful.

The word insofern (insofar) is an adverb placed in the main clause to set a restriction, and als (as) acts as a subordinating conjunction that explains that restriction. Because als is subordinating, it pushes the conjugated verb ist to the very end of its clause.

Could I use insoweit instead of insofern here?

Yes, insoweit and insofern are practically interchangeable when used to mean 'insofar as' or 'to the extent that'. You could perfectly well say Ich finde die Diät insoweit gut, als sie hilfreich ist with no change in meaning.

Why is als used here instead of a normal conjunction like dass?

The adverb insofern pairs specifically with als to mean 'insofar as' or 'to the extent that'. While dass (that) is the default conjunction for many dependent clauses, the fixed grammatical pair for this kind of restriction is always insofern ..., als ....

I thought als was used for past-tense events or comparisons?

You are right that als is typically used to mean 'when' for single events in the past (like als ich jung war) or 'than' in comparisons (like besser als). In this sentence, it takes on a third job: acting as the mandatory second half of the conjunction pair insofern ... als.