The 921 is a T10 wedge base bulb.
The Phillips is designed to utilize the reflector.
Some other T10 LEDs fire straight out not bothering with the reflector, but usually are much longer.
Some have hard driven 'Cree' emitters and can be extremely bright. I put Cree in quotes because these can be fakes.
I tried such a bulb in my 1156 ba15s reverse lights, and it might have been acceptable as far as reverse lighting from the driver, but it was NOT adequate for signaling drivers behind me that I was in reverse, especially backing out of a diagonal parking spot, where it was practically invisible, so I reinstalled the 1156 incandescent.
I imagine this type of bulb could work in some reverse light housings.
If you do get the phillips, I'd love to see a comparison shot to an incandescent 921, and hear your opinion.