True, but it's also one of the SMALLER airspaces in the world, which means that if anything starts happening, it's going to be over one way or another VERY quickly, and the Israelis are going to have to react quickly if they're to prevent this from happening.
How hard would it be for a lone fighter to escape visual or electronic detection?
The question is, "can it escape detection for a LONG ENOUGH time?"
If it penetrated Israeli airspace from Jordan, it could be almost all the way across Israel and over downtown Tel Aviv by the time Israeli fighters are getting airborne.
If it's an F-15E loaded out with FAST packs and drop tanks, it can actually fly across the Red Sea, across Egypt, into Libya, and out over the Mediterranean...and THEN step into a suicide run at Tel Aviv.
The plane wouldn't have the electronic warfare planes to provide cover and mitigate the radar.
It could use terrain masking, deceptive IFF emissions, and other techniques to delay detection or to slow down an Israeli response.