Quite, St Neots is the ideal option as St Neots is the only station of the three viable options (inc. Sandy & Hitchin) that is even remotely likely to justify ECML express stops for interchange.
Via Hitchin is the budget option, but you end up building a lot of new railway to get there anyway, and you then have a much longer (and slower) (and more congested) route. Via Sandy is admittedly the most direct, but it'll all be new build anyway, so you may as well try to:
a) hit a more viable mainline interchange (i.e. St Neots)
b) avoid reversals (i.e. continue through Bedford Midland)
c) Hit as many population centres as you can en-route (i.e. Cambourne)
...if the primary objective is an intercity 125mph line moving people between Cambridge and Oxford they will want as few stops as possible, so perhaps running through empty countryside is in fact the preferred option...
Not sure why St Neots - population 40,000 - would be a top spot for ECML trains to call, as opposed to Hitchin, population 33,000, with Letchworth -33,000 more - and Stevenage's 84,000 both close by, adding up to more than the 124,000 population of Cambridge. Which was presumably a key factor in via Hitchin being one of the two preferred options for further study.
Plenty of East Coast trains call at Stevenage, Alliance has just added plans to serve it to its London-Edinburgh open access proposal, First Group's rival London-Edinburgh open access plan already includes Stevenage calls, so it seems fair to say that this corner of north Hertfordshire is on the radar of TOCs one way and another. Make Hitchin an interchange with East West and giving access to frequent Cambridge connections and they might well favour it instead of its southern neighbour.
Distance-wise the old Bedford-Hitchin line was about 20 miles. The Hitchin-Cambridge line is probably about 26 miles.
Bedford-Cambridge via Sandy probably clocks in at a nudge over 30.
And reversals are not the end of the world if properly handled, e.g. align them with crew changeovers, so the new driver and guard are waiting ready to go as the train rolls in. It's not as if you're trying to achieve F1 pitstop times at what would be a place where lots of passengers would be getting on and off to make connections, whether Hitchin or anywhere else.