I don’t understand that comment. In relatively long range fire the shells of field artillery guns would also travel through the stratosphere. In shorter range fire in the lower register (ie below as opposed to above 45 deg elevation) neither would. The difference in trajectory is as mentioned in last post: 20th century field howitzers and ‘gun-howitzers’ often had provision for variable propellant charges, so could fire in the upper register at relatively close range without sending the shell up very high and introducing more variability due to upper air winds. That was a relative disadvantage to the flexibility of heavy AA guns as artillery pieces, since they typically didn’t feature variable charge ammo. But they were still quite useful as artillery, and in case of the German 88 and US 90 commonly used in that role where either a) not urgently needed as antitank batteries*, or b) it didn’t interfere with their use as AA guns**.
The trajectory height has nothing do with the issue of sights. That’s simply whether the individual gun was equipped with sights to accurately measure its azimuth wrt to fixed point within the line of sight (a so called aiming stake) from which its azimuth wrt to the unseen target could be calculated. All ‘indirect fire’ means is the target isn’t in the line of sight of the gun. So any gun with such a sighting set up can be used in indirect fire. Machine guns too can and also are/were, with appropriate sights. It’s possible a sophisticated AA gun wouldn’t have such individual sights, but strictly gets its firing info transmitted from an AA director. Then it would be more cumbersome (though still not impossible) to use it in indirect fire. But the German and US guns in general did have such sights. Again see the linked field manual for how a US 90mm battery would align its guns for indirect fire.
*eg. where the 88 gained its reputation as AT gun to the English speaking world, North Africa, the guns were often deployed well forward in strong points dug in as AT guns (eg. v the British ‘Battleaxe’ offensive in June 1941 which marked the first big shock of heavy British tank losses to 88’s). In general in the mobile phases of desert fighting targets didn’t stay still long enough for much indirect arty fire: even field pieces fired directly most of the time in the mobile desert battles. But in static phases and theaters, at times in North Africa, Italy most of the time, NW Europe campaign like Sep '44-Feb '45, there was more relative need for indirect fire and AA guns were often to the rear and used as indirect fire arty pieces.
**a lot of the US 90mm units were deployed (with British ones) to defend England then the port of Antwerp v V-1’s in 1944 whereas the field army didn’t need a lot of AA protection: a lot of them weren’t at the front to be used in secondary roles. And several 1,000 German AA guns (many >88 and/or captured weapons) were in the Reich defending against air attack. Those guns became a serious nuisance, as artillery as well as AT, when the Allied forces poured into Germany in April '45. But some Luftwaffe heavy AA units were always with the field army, where often used as artillery besides as AT guns.