I actually think Ivan's quite competent and based on a few stories it seems he's sensibly been putting out a few feelers for a while now.
Apparently he had a meeting with Tuchel last summer (before we got Sven) which went promising and he was eager to get the job if Wenger was to leave (unfortunately he didn't and signed a new contract - thanks Stan!) & Hasenhüttl confirmed that there was contact between an Arsenal rep and him last year.
The actual timing of the exact day Wenger announced it may have caught him by surprised but I think he's had a plan ready for a while now.