Actually, it's the opposite. Manchin is pretty well loved in West Virginia, with recent polls giving him
about 60% support (note how the comments in that article are mostly "lol west virginia is so retarded, that's why they like him" in typical smug lefty fashion). Those words are from a man who knows he's untouchable, not because he keeps getting opposed by ineffectual morons, but because he's at least somewhat concerned with representing his state's needs and enjoys high approval because of that. Like, I'm not gonna paint him as a saint or anything, but he knows which way the wind's blowing, and he's not about to do anything stupid to jeopardize his political future in the state.
Manchin is throwing down the gauntlet at Schumer's feet and
daring him to try and launch a primary against him. He knows exactly how it'll go since he won his last primary with 70% of the vote. He also isn't even up for reelection until 2024, so any threats are meaningless for at least another year, if not two. He has nothing to fear and everything to gain by continuing to hold the line against ol' Chucky Wucky. Since he only won his last election by about 20k votes, the state's increasingly turning red, so he'll need all the ammo he can get to convince WV to reelect him in a couple years. Luckily for him, Schumer's giving him everything he needs.