Just wondering why ACPA claims that long elbow creates more resistance then short elbow. (A sweep 90 degree bend (one meter radius)
creates as much pressure as nine feet of straight pipe; while a short 90 degree (250 mm radius)
creates the same pressure as three and one-half feet of pipe.)

may bee because they don't know what they are talking about.

It all matters on the concrete and the resistance of the material being pumped. Some material is like glue some like water

Well actually it shouldn't matter on principle only in magnitude. What I mean is direction change is a force, the shorter the turn radius the more force has been applied, creating more pressure, thus longer elbow should generate less resistance, then shorter elbow.

It doesn't say it creates more pressure it says as much as 9 feet of straight line, short radius   one metre radius elbow

A short radius elbow creates as much as 3.5 ft of line  a 250 mm sweep

Now multiply 250 x4 equals 1 metre

4 x 3.5 = 13.5 

So in answer to your question a short radius elbow creates more pressure the wording in the manual is a little confusing but it does not state one creates more than the other it states what each one creates in straight line pressure.

Its why long sweep elbows are used at the bottom of stand lines for high rise