How to shorten (replace) query of 2 CTE into 1 JOIN LATERAL

Hmmm . . . I think you want:

select t.*
from (select t.id, t.platform_order_number, v.platform,
             row_number() over (partition by v.platform order by t.platform_order_number) as seqnum
       from table t cross apply lateral
            (values (case when platform_order_number LIKE 'B2C%' then 'B2C'
                          when platform_order_number LIKE 'B2B%' then 'C2C'
                          else 'C2C'
                     end)
            ) v(platform)
      ) t;

In general, window functions may not behave as expected in a lateral join. They only work on the rows processed in the lateral join (often just one row) rather than spanning the original table.