I haven't seen enough evidence yet that Nichols went to the Philipines at all.
Even if he did - so what? I went to the same general area for a vacation, so why couldn't he?
Also, can you point to just when a fertilizer (amonia nitrate) enhanced bomb was used to destroy any building before OKC ?
Yup! The Texas City Disaster, 1947. A boatload of ammonia nitrate sitting in the harbor caught fire. When the cargo finally detonated it wiped out a large portion of the city. Ironically, the detonation ignited another ship with the same cargo, which also eventually detonated causing further damage.
Lots more documentation here.
But wait - there's more! A few months later, a similarly-laden ship ignited and detonated in France, also causing significant architectural damage. Other instances involved major ANFO-type explosions, but were fortunately small enough and/or far enough from buildings to limit the damage: Repauno, January 14, 1916, Oakdale, September 15, 1916, Nixon, March 1, 1916, Red Sea, January 23, 1954, Traskwood, AR, December 17, 1960, Australia, August 30, 1972, Pryor Creek, OK, January 17, 1973.
And all these were accidents, featuring large amounts of legitimately transported or stored quantities of ammonium nitrate, where burning liquid fuels were somehow mixed in (doesn't take much) and the hot concoction seriously confined - exactly what McVeigh did, only intentionally.
This mixture is not uncommonly used on a small scale on farms (which tend to have the ingredients on hand) for removing stumps and rocks.
That McVeigh may have been the first to do it as an intentional large-scale criminal act likely has more to do with (a) not many people set out to do that kind of evil, and (b) there are more convenient methods.
To the thread's topic: if such ANFO detonations have occured accidentally on numerous occasions, there is no need for Nichols to have travelled halfway around the world to find out how to do such a simple & cheap process intentionally, particularly when he likely was taught it by father or boss using items in the family or business' barn.
You are talking apples and oranges or at least pop gun and cannon. Scaling a bomb is not as simple a process as you seem to want to make it out to be.