Factor, 30 29 bytes

Saved 1 byte thanks to @Bubbler

[ 1 rot neg <range> product ] 

Try it online!

Takes k (which -th factorial) and then n. First we push an n onto the stack, then rot makes the stack look like n 1 k. neg negates k, which will act as the step for a range from n to 1 (made using <range>). Then product finds the product of that range, and that's it.

Factor + math.unicode, 30 29 24 bytes

• Saved 1 byte thanks to @Bubbler
• Saved 5 bytes thanks to @chunes!

[ 1 rot neg <range> Π ] 

Try it online!

Takes k (which -th factorial) and then n. First we push an n onto the stack, then rot makes the stack look like n 1 k. neg negates k, which will act as the step for a range from n to 1 (made using <range>). Then Π finds the product of that range, and that's it.

Factor, 30 bytes

[ neg 1 swap <range> product ] 

Try it online!

Takes n and then k (which -th factorial). neg negates k, which will act as the step for a range from n to 1 (made using <range>). Then product finds the product of that range, and that's it.

Factor, 30 29 bytes

Saved 1 byte thanks to @Bubbler

[ 1 rot neg <range> product ] 

Try it online!

Takes k (which -th factorial) and then n. First we push an n onto the stack, then rot makes the stack look like n 1 k. neg negates k, which will act as the step for a range from n to 1 (made using <range>). Then product finds the product of that range, and that's it.