Looks like a spike in Tetr.io is just a name to tell when you get enough garbage to send, to make you understand that you will launch a good amount of it.
Here's a table chart from Tetr.io wiki with its' legend :
An illustration of the combo table in use, as well as an explanation of the "B2B Chaining" mechanic. Yellow cells indicate the attack shown is strong enough to cause a "spike", red cells indicate that the garbage sent would be enough to cause a topout if garbage is not countered, and purple cells indicate an attack which is large enough to cause a "spike KO"
As you can see, a combo becomes a spike just by getting bigger, so I don't think there's more about this (plus I didn't found anything more about this term on the wiki or anywhere else).