Spontaneous / open fermentation and no airlock are two different things IMO.
If you let the wort open to the air, that's open to insects, bacteria, yeasts in the air. The result might be interesting but not necessarily predictable.
If you let a cap on the fermenter, that will prevent insects, bacteria, yeasts in the air to get in touch with the wort. The pressure created by the fermentation will make the CO2 escape but the movement should be one way, a bit what happens in hospitals where they have positive pressure rooms, what happens in your kettle when the vapour lifts the cap and makes it grumble, or what happens in your Fido jar (or in a Grolsh bottle) when excess pressure is vented outside by some leakage, because the CO2 overcomes the spring of the cap (that makes Grolsch bottles very safe in respect to the risk of explosion. They will not explode, they will vent).
A decently heavy cover on a silicon gasket will decently protect your beer and will "leak" some CO2 during fermentation.
An airlock is a more elegant and precise solution, but it's not strictly necessary. Just like you can ferment Sauerkraut with or without an airlock and without infections, so you can ferment beer with or without an airlock. It's the leakage of your cap which will vent the CO2.
Yet, if the CO2 displaces even a little bit the cap and the "seal" is not maintained, then you could have air entering in.