Does Shuckle really produce Rare Candies?

No, but Shuckle can turn a Berry into Berry Juice

A disassembly of Pokémon Crystal is available on GitHub, and nowhere in it is code for having Shuckle turning Berry Juice into a Rare Candy.

However, this rumor probably has its origin in the fact that Shuckle can turn a Berry into Berry Juice after the player has visited Goldenrod City. This section of code includes a function that runs after the player has visited Goldenrod City, then has a 1/16 chance of a Shuckle holding a Berry turning that into Berry Juice.

ConvertBerriesToBerryJuice:
; If we haven't been to Goldenrod City at least once,
; prevent Shuckle from turning held Berry into Berry Juice.
    ld hl, wStatusFlags2
    bit STATUSFLAGS2_REACHED_GOLDENROD_F, [hl]
    ret z
    call Random
    cp 6 percent + 1 ; 1/16 chance
    ret nc
    ld hl, wPartyMons
    ld a, [wPartyCount]
.partyMonLoop
    push af
    push hl
    ld a, [hl]
    cp SHUCKLE
    jr nz, .loopMon
    ld bc, MON_ITEM
    add hl, bc
    ld a, [hl]
    cp BERRY
    jr z, .convertToJuice

.loopMon
    pop hl
    ld bc, PARTYMON_STRUCT_LENGTH
    add hl, bc
    pop af
    dec a
    jr nz, .partyMonLoop
    ret

.convertToJuice
    ld a, BERRY_JUICE
    ld [hl], a
    pop hl
    pop af
    ret

No, Shuckle could never turn Berry Juice into a Rare Candy. That is just a rumor.

Here's a YouTuber going through the disassembled code to debunk it.