Again, like a dead horse, I'll repeat what makes Hendricks different: lack of pitches.
If we're on the subject of two pitch pitchers then Jose Quintana is one. On his career he's throw 51.9% 4 seam fastballs and 23.1% curves. Gerrit Cole has thrown his 4 seam 62.2% and his slider 16.7%. Taijuan Walker threw his 4 seam 64.5% and his splitter 15.5%. Shelby Miller until this year had been 60% 4 seam 20% curve. To throw up the Jordan Zimmermann comp again, on his career he's throw 63.9% 4 seam, 20.5% slider. Chris Archer was 44.0% 4 seam, 39.1% slider last season clearly his best season. In other words, this is fairly common.
And because I'm sure the next comment will be their 2 pitchers are better than his 2 pitches.
Hendricks
Sinker - .281/.338/.407 116 wRC+
Change - .164/.191/.263 32 wRC+
Third time through a line up - .311/.360/.451 .368 BABIP
Jose Quintana
4 seam - .275/.331/.412 117 wRC+
Curve - .200/.231/.309 59 wRC+
Third time through a line up - .288/.335/.429 .330 BABIP
Gerrit Cole
4 seam - .256/.316/.352 97 wRC+
Slider - .179/.213/.276 45 wRC+
Third time through a line up .255/.320/.377 .305 BABIP
Taijuan Walker
4 seam - .242/.304/.416 112 wRC+
Splitter - .294/.319/.401 112 wRC+
Third time through a line up .242/.303/.368 .276 BABIP
Jordan Zimmermann
4 seam - .263/.311/.408 107 wRC+
Slider - .228/.263/.335 73 wRC+
Third time through a line up .257/.309/.403 .288 BABIP
Shelby Miller
4 seam - .225/.302/.363 97 wRC+
Curve - .229/.276/.354 87 wRC+
Third time through a line up .272/.350/.423 .300 BABIP
Chris Archer
4 seam - .265/.363/.396 128 wRC+
Slider - .187/.231/.288 56 wRC+
Third time through a line up .233/.300/.355 .277 BABIP
Hendricks' primary offering(sinker) could use some work but it's not some abomination. Incidentally it's not even the worst here. Archer had the worst primary offering on his career. On the other hand, Hendricks secondary offering(change) was the best out of all of these pitchers and one of the better in baseball. For instnace, Sale and Kershaw's sliders are 30 and 31 wRC+ respectively. The difference between their OBP and Hendricks' the third time through the line up is almost entire because of BABIP. The differences are Quintana(+25/+38 OBP/BABIP), Cole(+40/+53), Walker(+57/+92), Zimmermann(+51/+80), Miller +10/+68) and Archer(+60/+91). If that comes down as you would expect, so does Hendricks OBP.
If you want to believe there's a ceiling in place on Hendricks ability because he's a 2 pitch pitcher who as of yet hasn't pitched well the third time through a line up then that's your call. But that MO fits on a number of pitchers and some of them are considered quite good. The list of 62 seasons I posted that matched what Hendricks did this past season wasn't full of many average players. So guess I'll take the over.