"to" can introduce an infinitive, sometimes with the meaning of purpose, and it can also be a preposition. When it is a preposition, it is followed by V-ing.
In It is equivalent to saying that... "to" is a preposition, and the sentence means that something is the same as saying something else, for example:
- Walking out on your wife is equivalent to saying you don't love her anymore. (Walking out on your wife = saying you don't love her anymore)
When you establish the equivalence between two things, if one of those things is a verb it has to be a GERUND.
For the infinitive to work, we can think of other adjectives, for example:
It is reasonable to say that this is a good method to learn English.
In this case, "to say that..." is the infinitival subject of the sentence.