What you are seeing in these videos is a test of this Evergreen Supertanker plane.   To get permission for it to be used they have to run a series of tests and demonstrations.   The early tests are done at altitude to ensure that the jetison method works correctly and also to ensure that the changes in weight/balance occur in a way that is controllable; dumping that amount of water quickly will have an affect on those elements which in turn effect the controlability of the aircraft.

Once they understand the basic parameters they then have to work out the best way to use the plane.   Smaller, more manouverable, aircraft can get close into fires.   This has a range of risks because the fire is heating the air causing unstable updraughts which can affect the controllability and several crashes have occurred when planes have been caught in air currents and the pilots have reacted in such a way as to overstress the airframe and it has broken up in flight.   It’s worth bearing in mind that these situations are way outside the original design specification of the aircraft.   The 747 simply doesn’t have the manouverability of those smaller aircraft so it simply can’t fly as low and relies on the enormous load it carries.   The smaller aircraft have much smaller loads and therefore have to try and deliver their water/retardant with much greater precision hence flying into dangerous situations.   So the 747 flies higher but dumps much, much more.   But when used in anger the 747 will be flying at lower altitudes than the tests seen here.