I don't know where Motley Fool got their short interest figures but the Nasdaq site shows short interest on the 7/31 report as 7,354,161. The previous bi-monthly report dated 7/13 showed short interest of 12,401,195 which is the figure that corresponds to what Motley is using. Motley's article is dated August 10 but it may be that the more recent report on short interest dated 7/31 had not yet been published.
The little crash in AGNC that might have been related to a concern about speeded up repayments didn't happen until the Aug. 6/7 trading so it seems pretty certain that about 5m shares that had been short were covered BEFORE that minor crash (sorry, shorty!).
Shorts probably took the opportunity to do some covering and take advantage of the pullback of aug 7. Given that insider/president bought largish shares at 32.75 (which must've been on the 7th), I wouldn't guess that concerns about repayments and/or interest spreads are well founded but who knows.