I was comparing Grimsby times with Grimsby times.
You're hardly being fair though; if you are adding the ten minutes of Cleethorpes to Grimsby onto the "via Doncaster" journey time then it seems a little skewed to make the "via Lincoln" journey seem faster by only starting that journey at Grimsby.
For me, it seems pretty even to get from Grimsby to London via Doncaster/Lincoln if you were doing it in ideal circumstances - the only main difference will be how much connection there'll be at Lincoln onto the bi-hourly LNER service (i.e. between the two Lincoln - Newark trains) compared to the connection at Doncaster (which gets around four trains per hour to Kings Cross, so the connection time isn't bad).
I get that people want to promote the idea of an improved link and all that, but I don't think that "hourly service to Lincoln to connect onto a bi-hourly service" is going to be transformational for Grimsby passengers when compared to the existing "hourly service to Doncaster to connect onto one of four London departures per hour".
If anything, the Nottingham link should be of much better benefit to Grimsby passengers (esp given how poorly Nottingham is linked to places north of it).