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 spike appears when a combo grows large enough, 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).
It's still important to differentiate combo and spike, as the first one is the count of successive attacks, and the latter is the total amount of garbage pilled up from all these successive attacks.
This is a video from a theknightD2 comment, where we can clearly see the combo count going from 1 to 1, while the spike goes up from different values due to the different attacks performed.