Despite removing and greasing it, my pedal still won't rotate - I can't figure out why

Yesterday out cycling on muddy farm roads, my right pedal seized up and stopped turning (the crank turns fine, but the pedal itself doesn’t turn to stay parallel with the ground, making cycling difficult because my foot keeps coming off). I assume it got grit/mud in it – at one point the road was flooded and the bike got immersed up to the top of the chainring – but I’ve removed the pedal, rinsed, dried and oiled the threads, and replaced it, but it’s still really stiff. I’m fairly confused as to why – there was no visible mud even before cleaning. Any thoughts? I used a generic bike oil – would getting penetrating oil help?

It’s a new bike (Sirius X 3.0) – this is my 10th or so ride, no issues hitherto.
Edit: as per the comments/answers, I realised it is the locknut – it doesn’t appear to be removable as you can see in the image, and WD40 doesn’t appear to be helping. I suppose taking it back to the shop is the only thing to do if the pedal can’t be disassembled.