Why? After they've spent all that money on modernising their Mark 3s with plug doors etc. I see no reason why they'd be in a hurry to replace them, and their advertising for the Mainline route is centred on the spacious Mark 3 ''Mainline'' trains, as opposed to their Clubman DMUs. This 68 deal (haven't seen any formal evidence yet) merely suggests the loco hauled trains are here to stay (remember there was a tender for 15 diesel locos last year, so this is probably it).
The advertising may be centred on the Mk3 sets, but the Mainline brand applies to both train types, with the 168s being reliveried into silver to match the Mk3s, and the two types are identified separately on the timetables as M (168) or MS (Mk3 set).
When Chiltern's parent company puts out a tender for new locomotives to run Chiltern's loco hauled operations I think it's a pretty clear sign that they do not expect to receive any more DMUs to replace its mk3 stock anytime in the foreseeable future. If and when the 170s do start to get cascaded I can think of a few places that will need them more than Chiltern.
The TPE situation is a bit different, the units came off lease end of, and Chiltern snapped them up.
I think you can probably forget all about that tender for locomotives. It was issued getting on for two years ago and nothing ever happened by way of an order, indeed the locos were supposed to be in use by this December and any tender offers that were made will have lapsed by now anyway, with the new EU rules on diesel engine emissions coming in at the end of the year.
With the deal for the 170s agreed, I can't see Chiltern rushing to convert more Mk3s anyway, so they will just need a small group of locos and DRS is probably happy to have guaranteed work for part of the 68 fleet.
The possibility that Chiltern might lease 68s, whether directly or from DRS, was discussed in many places back in 2012 when they were ordered, so it's hardly the most astonishing development if it is true, though I can't see the 'Chiltern gaffer' sharing too much more information with the original poster's father, should their paths cross again in the future.