I think she meant that the preposition "sotto" agrees with "a" in written Italian (in contrast to prepositions which agree with "da", "nel" etc.) "Sotta alla" derives from the definite article (la) which is attached to the original "a", thus creating the "alla". I think you should read it literally, I don't think it's an idiom.
According to Treccani 'sotto' can be used with or without the preposition 'a': "...è invece frequente con sostantivi (s. al letto, s. al pavimento, che si alternano con s. il letto, s. il pavimento)." Treccani makes no distinction as to whether the one usage or the other is more common in written Italian. Most of the examples are without 'a', except those where 'sotto' is used with personal pronouns, in which case 'di' is generally required after 'sotto'.
If I could understand the Italian, I think the answer is here! Seems that sopra can take 'a' or 'di' http://forum.wordreference.com/showthread.php?t=1058378
I believe I start to get it. Previously I only had 'sotto voce' as a reference. But I took the time to look up the German translation and I gather that 'sotto' does not mean 'under' but rather something like 'down', 'low', I.e. it is an adverbial expression and not a preposition, so we are rightfully punished by duolingo for not using the 'alla' for the spatial reference.
SOTTO O SOTTO A?
Sono corrette entrambe le forme:
Il gatto si era nascosto sotto il tavolo.
La valigia è sotto al letto.
Prima di un pronome personale tonico, la sequenza preferibile è sotto di:
Con la terra sotto di me / l’aereo sembra fermo (Nek, Con la terra sotto di me).