Government has to pass vote of confidence to form. PiS and its allies even combined don't have the numbers per preliminary estimates. Where as estimate is the main opposition and their supporting coalition of parties will have the numbers.

There is no way opposition is going to do the incumbent government the favor of refraining from voting down the PiS attempt to form government. If PiS and their one likely partner party try to form government, estimate is, attempt will crash and burn by geting voted no confidence by Parliament.

This is how things go in multiparty democracies and coalition governments. It doesn't matter who has plurality. What matters is "can you pass the government forming vote of confidence". Prime minister could be from the smallest party in Parliament as long as he can gather Parliament's confidence.

Edit: minority governments do happen in multiparty Parliaments, buy those happen at the leave of the Parliament. Meaning the minority is allowed to pass confidence. Either by directly getting support votes or depending voting system by parties voting empty. Some countries confidence votes is "there is confidence, unless majority actively votes yes on no confidence proposal". Making voting empty a tacit not enthusiastic signal of confidence. "We won't actively vote for you, but we won't either vote you down". Leaving government standing.

Minority government can not form against active resistance of majority of Parliament.