It's worth sitting with this, because founders resist it and the resistance is what costs them money.
A plaintiff does not need evidence to file. They need a filing fee and a lawyer willing to sign. The screening that separates good claims from bad ones happens later — at a motion to dismiss, at summary judgment, sometimes not until trial. Each of those gates costs money to reach.
So the question "do I have to take this seriously" has a boring answer. Yes. Always. Even when the claim is invented, even when you have receipts, even when the person suing you knows better. The summons has a deadline on it and the deadline does not care whether the allegations are true.
What being right does buy you: better odds at every gate, more leverage in every negotiation, and a cheaper exit. What it does not buy you: the ability to skip the process.
The corollary founders miss. Being right is not free either. A meritless case and a meritorious one cost roughly the same to defend through discovery, because discovery is priced by volume of documents and hours of lawyer time, not by who's telling the truth. This is the single most counterintuitive fact about litigation and the one that wrecks the most budgets.