Blurry background graphics are hardly indisputable in-universe fact. Even if it was the intent at the time for the 1701-A to be transwarp-capable, that the "trans-" prefix vanished and several statements from the producers since have dubbed transwarp a failure pretty much spells it out.
I guess Scotty's "'Let's see that she's got' said the captain... then we found out, didn't we?" in STV could even be taken as in-universe proof of TW's failing. (and before you all say "but it would explain the quick trip to the galactic centre", I remind you it was well within the way warp speeds were depicted in the TOS Trek - edge of the galaxy in WNMHGB and "By Any Other Name", centre in "Megas Tu" ...)