"Amortizar" here has one of its standard meanings: "recuperar o compensar los fondos invertidos en alguna empresa" (DLE def. 2)
https://dle.rae.es/?id=2QNpMF8

It means "sacar rentabilidad", treating time here as an investment.

The quotation from Llorente has been clumsily incorporated into the sentence, as reflected by the singular verbs (disminuye, se amortiza) with a plural subject (retrasos). In Llorente's original, which Robert has usefully quoted, the subject is "tiempo": the time teachers have to spend learning to use the digital blackboard and preparing teaching material with it. This is seen as an investment up front which pays off over time. The idea is that on the one hand this time invested by the teacher is reduced through training — as they are trained to use the media they get quicker at it — and once they've made that investment, they can capitalize on it by using it multiple times: "al aumentar el número de sesiones". So the more teaching sessions they use it for, having mastered it, the more they profit from the time they spent learning to use it. That's the idea of "amortizar": recouping or capitalizing on an investment.

You could say "is/can be recouped by increasing the number of sessions". I like "capitalize on" here, but it's awkward in the passive ("can be capitalized on by..."). However, you could make this part active: "and moreover teachers can capitalize on it by increasing the number of session". Another possibility might be to use "pays off" for "se amortiza": "and moreover it pays off when/if/as the number of sessions increases".