TL;DR: Infinitive suffix "-ти" and 3rd person singular present "-тъ" are essentially different.
They could reduce independently in Slavonic languages, but if they are both reduced, this seems to be just a coincidence.
In his "Slavonic Grammar with Correct Syntax" (1619), Meletius Smotrytsky marks 3rd person singular present suffix
-тъ (pronounced with short
So, the verb to read, "читати"
[t͡ɕɨ-ta-ti] conjugates to "чтєтъ"
Here's the scan of Smotrytsky's work
Links: exact page, title page.
In modern Slavonic languages, indeed, these suffixed often reduce.
Ukrainian (my native language): the infinitive suffix remains the same, while the 3rd person singular present suffix reduces: "читати"
[t͡ɕɨ-ta-tɨ] → "читає"
[t͡ɕɨ-ta-ʲe] or even "чита"
[t͡ɕɨ-ta] in Western dialects.
On the contrary, the modern Russian reduced suffix in the infinitive form while it retains 3rd person singular present suffix: "читать"
[t͡ɕi-tatʲ] → "читает"
So, as we can see, the distinction between Ukrainian and Russian suggests that both suffixes may retract independently.