Flop is harsh. It's been noted on numerous occasions that tight end is a difficult position for a rookie to come in and contribute right away. Both players had injuries following no offseason programs, limited training camp, and no preseason. They then came into an offense that was run first, run second, and run third if it could be justified, and had a QB who really only clicked with two receivers.
The acquisition of Henry and Smith certainly doesn't say they're confident in them making a leap to pro-bowlers in year 2, but we know three things: 1) Belichick had an unusually high amount of money to work with this offseason. 2) He seems to really like 2 tight end sets if he can get the horses to run it. 3) He's had very positive things to say about both vets prior to this offseason. He had the opportunity and means to add two great players to a position grouping he wants to feature. There's a better than average chance that he signs one or both of them, even if the rookies had shown something more than they had.
Does this mean they're guaranteed to improve? Definitely not. Does this mean both make the roster this year? Again, no. But when you want to run two-TE sets, you need more than two tight ends that you feel confident in, and the trade of Izzo has left basically Keene and Asiasi as the only two competing for the depth spot at what has become a critical position for the offense (unless others are seeing more in LaCosse than I am). Looking at all the circumstances from last year and writing them off as flops is premature, IMO.