On whether to buy the purifier anyway: it is not wasted, it is just second in line.
What a purifier genuinely does well is reduce the background and clear a room after an event. So the useful arrangement is source control while cooking, and a purifier to bring the number down afterwards and to handle the general baseline.
What it does badly is compete with an active source. A purifier running while you fry is trying to filter a room that is being filled faster than it can clean, and that is why people conclude purifiers do nothing.
If you buy one:
Size it for the room using the stated clean air delivery figure, and buy one rated well above the room rather than exactly at it, so it can run on a low quiet setting.
Put it in the room you spend time in, not the kitchen. Kitchen air needs to leave the building, not be filtered.
Budget for filters. The running cost is real and it is the thing that determines whether it is still in use in two years.
And keep the monitor. It is the reason you asked the right question instead of buying the purifier and wondering why the readings did not change - that is a genuinely good outcome for a cheap sensor.