The players that get reps are rookies first and foremost, suggesting that they would give valuable reps to an UDFA over a rookie who was going to make the team over the UDFA is illogical. Kline and Devey are getting reps over the rookies because they have outperformed them and are likely to hold more significant roles on the team.
That assessment is only partially correct. BB is a good coach and will play them and see if his PS groomed UDFAs have progressed, and how far. It does NOT necessarily mean they have outplayed the more highly regarded draftees. This is playtime games, almost scrimmages, not the real games, so he can afford to do so.
Look at the situation after last season. BB had five regulars and one T/G reserve, plus a bunch of UDFA scrub/projects that Scar was working with. The line was good. Good enough to get to three consecutive AFCCGs and a SB invitation.
However, two of the regulars were now over thirty, and would need replacement within a few years. In addition the line had seemed to spring a leak, and Brady was sacked more often then ever before. Perhaps that was because the planned 2TE Offense disappeared amid arrests and injury, maybe not. Behind them were only the problematical UDFAs.
Belichick knew there was no genuine urgency, still he needed to increase the talent level of the reserves, and start grooming future successors. He drafted two good ones for C and RG, and a third guard with hard nose talent, who slid. Plus it appears one or two of the UDFAs are showing promise.
Whether the youngsters start now, mid-season or next year matters little. The long term Offensive line looks fine now, with maybe with another player or two for spice.