https://dev.mysql.com/doc/refman/5.6/en/column-count-limit.html

There is a hard limit of 4096 columns per table, but the effective maximum may be less for a given table. The exact limit depends on several interacting factors.

• Every table (regardless of storage engine) has a maximum row size of 65,535 bytes. Storage engines may place additional constraints on this limit, reducing the effective maximum row size.

...

These are engine-independent MySQL limits. As you say the MEMORY engine specific manual pages do not seem to specify any additional restrictions for size or count.

From This MySQL link

There is a hard limit of 4096 columns per table, but the effective maximum may be less for a given table. The exact limit depends on several interacting factors.

• Every table (regardless of storage engine) has a maximum row size of 65,535 bytes. Storage engines may place additional constraints on this limit, reducing the effective maximum row size.

...

These are engine-independent MySQL limits. As you say the MEMORY engine specific manual pages do not seem to specify any additional restrictions for size or count.