It is a major stretch. Thanos, in that series, has already downed King Thor, Galactus, Celestials and so on. Things that are at least if not higher than skyfathers.
I am behind reading these things but it seems like Thanos has developed his own power over time and added to it. This is hardly normal Thanos going at the moment.
That said, Surfer with Mjolnir should not be on the same level at all. Given what he has downed so far.
True, but we are talking about the power of King Thor mixed with power cosmic.. perhaps the sum is greater than the parts? Will have to wait for the next issue to see..