Were K2-141b and WASP-63b selected as the first two exoplanet atmospheres for JWST to look at during its first year of observations? If so, why?
The list of JWST approved programs for Cycle 1 (12 months long, starting after the 6 month long commissioning phase following launch) is available here. If you unfold the Exoplanets and Disks science category, you will see the programs, along with the links to proposal abstracts and target lists.
Looking at the proposal titles only, the exoplanet targets for atmospheric studies  include:

HD 189733b (2 proposals)
WASP-121b
Gliese 486b
GJ1214b
TOI-421b
55 Cnc e
HD 80606b
WASP-166b
K2-141b (2 proposals)
WASP-127b
Kepler 51d

The Espinoza et al. proposal is proposal number 2159 is one of two to study K2-141b. So K2-141b and WASP-63b are just two of the exoplanets that will be studied by JWST in the first cycle, with others likely in following cycles.
In the overall summary of the JWST Cycle 1 proposal process that was made available to the community, exoplanets accounted for 928 out of the 3500 hours allocated to small programs (slide 7). The exoplanets to be studied are chosen by the individual investigation teams that submit the proposals based on what particular aspects of the atmosphere they are trying to study. These proposals are evaluated by the various exoplanet subpanels of the time allocation committees and by Space Telescope Science Institute (STScI; who operate JWST and HST) staff for scientific merit, technical feasibility, capabilities of the proposing team and optimal use of JWST, a process which is common to almost all competed observing facilities. (The science panels are supposed to only consider the scientific merit, the technical feasibility is handled by STScI staff; this process has been improved by STScI for recent HST and all JWST Cycles by employing Dual Anonymous Peer Review where the panel doesn't know the identity of the proposers and the proposers don't know the membership of the review panels)