This is just wrong. Pitch when used as a verb can - and sometimes must - be used without "to" ("He pitched me a new startup idea" vs *"He pitched to me a new startup idea"). As a noun it does need "to".
He's correct in this context though, because it is ambiguous as written. It has a great many meanings, in fact: you could mean to take your clients and throw them at a high velocity. It could mean to take some of your clients and heave them into a pile. It could mean to throw your clients into a fermentation vessel. You might mean to suspend your clients by poles to make a temporary shelter out of them for your camping trip.
"Pitch" when used as a verb without "to" has many more possible meanings than when used with "to"—the "to" clarifies the subject-object relationship. Without "to," the the clients could either be the object or the subject, for example it could be the clients "being pitched to" (they are the object of the verb) or "being pitched" (where they are the subject). Adding "to" clarifies and locks down the meaning of the sentence, and a sentence without ambiguity is generally the better one.
Incidentally, adding "an idea" at the end ("Pitch your clients an idea") also clarifies the subject—the subject of the pitch is clearly the idea, since it comes after the object and that's just the ordering rule that English likes; you could also reverse them, but then you'd need the "to" again to clarify the relationship, eg: "Pitch an idea to your clients".
Thus, as it stands, "Pitch to your clients" is indeed the more correct and clear English sentence.
However, given English is not the writer's first language, he's doing extremely well and should be commended. English is an extremely difficult language with a myriad of special cases and nuances, and it takes many years of dedicated practice to master them. We can only be supportive and educational, and hopefully this was.
That only sounds unnatural because of ordering (which is a little like old-style English). Reordered, it reads just fine:
"He pitched a new startup idea to me"
The "pitch [object] to [party]" usage is the older/more established, I think, although recent (American) usage has somewhat shifted to "pitch [party]".
It's not a big deal either way. Maybe ivix is from the UK or somewhere with different usage.
"Pitch" when used as a verb without "to" has many more possible meanings than when used with "to"—the "to" clarifies the subject-object relationship. Without "to," the the clients could either be the object or the subject, for example it could be the clients "being pitched to" (they are the object of the verb) or "being pitched" (where they are the subject). Adding "to" clarifies and locks down the meaning of the sentence, and a sentence without ambiguity is generally the better one.
Incidentally, adding "an idea" at the end ("Pitch your clients an idea") also clarifies the subject—the subject of the pitch is clearly the idea, since it comes after the object and that's just the ordering rule that English likes; you could also reverse them, but then you'd need the "to" again to clarify the relationship, eg: "Pitch an idea to your clients".
Thus, as it stands, "Pitch to your clients" is indeed the more correct and clear English sentence.
However, given English is not the writer's first language, he's doing extremely well and should be commended. English is an extremely difficult language with a myriad of special cases and nuances, and it takes many years of dedicated practice to master them. We can only be supportive and educational, and hopefully this was.