What necessitates "the sounding of the Echo" is unknown. It is known it was sounded after Silvertear; however, primals were still summoned prior to the event. It was just a rare event thanks to both Midgardsormr's seal on the lake and the Eorzeans keeping the beast tribes' crystal stocks low through trade. Garlemald's attack nixed both of those, leading to the proliferation of summons.

Furthermore since Garlemald just happened on the Burn when it marched on Othard... we don't know how old it actually is. It could predate the Sixth Astral Era.