Your point highlights the way CERA should be used.
One can not just blindly look at each catcher's CERA on a team, you have to look at the pitcher by pitcher comps. Of course some comps will have wildly different sample size innings pitched that make some comps unreliable.
Here's a look at 2018 OPS against by pitcher:
(Only pitchers with 80+ PAs against listed)
Porcello
.700 w Leon (714 PAs)
N/A w Vazquez
N/A w Swihart
No Comp: Porcello has only been caught by Leon.
Price
.562 w Leon (232)
.780 w Vaz (394)
N/A w Swihart
Leon is the clear better choice w Price.
Sale
.497 w Leon (406)
.572 w Vaz (160)
N/A w Swihart
Leon is the better choice with Sale.
Rodriguez
.408 w Leon (43)
.687 w Vaz (401)
N/A w Swihart
Leon is the better choice, but the sample size differential is large.
Johnson
.683 w Leon (105)
.843 w Vaz (128)
.731 w Swihart (144)
Leon is slightly better than Swihart. Sample sizes all close to even.
Velazquez
.761 w Leon (83)
.738 w Vaz (134)
.818 w Swihart (83)
Vaz is slightly better than Leon. Sample sizes pretty close.
Pomeranz
.961 w Leon (93)
.948 w Vaz (127)
.723 w Swihart (72)
Swihart clearly does better with Pom.
Barnes
.591 w Leon (96)
.493 w Vaz (113)
1.049 w Swihart (33)
Vaz does better with Barnes.
Kelly
.886 w Leon (91)
.429 w Vaz (108)
.531 w Swihart (36)
Vaz does way better with Kelly than Leon. Swihart does well, too.
Hembree
.635 w Leon (84)
.748 w Vaz (107)
.657 w Swihart (34)
Leon does better than Vaz. Swihart does well.
Kimbrel
.587 w Leon (107)
.610 w Vaz (98)
(Just 6 PAs with Swihart .750)
Leon does slightly better than Vaz.
Wright
.396 w Leon (16)
.645 w Vaz (145)
(Just 4 PAs with Swihart 1.167)
Leon does better, but with way less PAs against.
Eovaldi
.703 w Leon (131)
N/A w Vaz
N/A w Swihart
No Comp
Workman
.856 w Leon (64)
.454 w Vaz (35)
.465 w Swihart (22)
Vaz does better. Swihart does well.
Thornburg
.744 w Leon (72)
N/A w Vaz
1.230 w Swihart (18)
Leon does better, but sample sizes are small.
Brasier
.480 w Leon (55)
N/A w Vaz
.358 w Swihart (23)
Swihart does better, but the sample sizes are small.
Overall, it looks like Leon does way better with our top 4 starters by PAs against with Porcello only using Leon as well.
8 pitchers do better with Leon
4 do better with Vaz
2 do better with Swihart
2 only are caught by Leon
For what it's worth, here are the overall CERA numbers:
3.27 w Leon
3.76 w Vaz
4.42 w Swihart
5.40 w Butler