I think the KEYMAP macro that you are relying on has changed since that guide was written. This change might have been unintentional by the author (i.e. a bug) but it can be fixed in multiple ways, either by switching around the keys in the keymap config as you did, or better by editing the KEYMAP_PCBUP macro in keymap_common.h so it looks like this:
#define KEYMAP_PCBUP( \
K00, K01, K02, K03, K04, K05, K06, K07, K08, K09, \
K10, K11, K12, K13, K14, K15, K16, K17, K18, K19, \
K20, K21, K22, K23, K24, K25, K26, K27, K28, K29, K2A, \
K30, K31, K32, K33, K34, K35, K36, K37, K38, K39, K3A \
) { \
{ K00, K01, K02, K03, K04, KC_NO, K05, K06, K07, K08, K09 }, \
{ K10, K11, K12, K13, K14, KC_NO, K15, K16, K17, K18, K19 }, \
{ K20, K21, K22, K23, K24, K35, K25, K26, K27, K28, K29 }, \
{ K2A, K30, K31, K32, K33, K34, K36, K37, K38, K39, K3A } \
}