Is microfiber in bed linen the same as 600 thread cotton?

No. Microfiber thread count cannot be compared to the thread count in cotton. A 1500 thread count bed sheet of Microfiber might not be half as heavy in weight or thick enough as a 600 thread count cotton sheet, but it will be far softer than a cotton 600 count.

It is similar to comparing Apples and Oranges. Microfiber has a thread count of it's own and tends to remain thinner, although it gets softer, with higher thread counts. Cotton on the other hand, tends to get thicker and heavier as the thread count goes higher, so a 1500 count sheet in cotton might weigh like a blanket.