That sounds pretty clear for me. I mean of course there is still a chance that Bashiok misinterpreted the question but I highly doubt it. The guy is asking for "My" 1-20 lvl toons. That implies that they already exist and are not newly created on a lapsed account. Also the next answer concerning characters over lvl 20 underlines this interpretation.
To play with some logic and sense. When they lock characters that you played while subbed and leaving you only be able to play characters that you rolled while unsubbed. This f2p character would automatically get an upgrade with your next sub, meaning you wouldnt be able to play it, when sub runs out again. In this case you would have to reroll your f2p again. So basically, they would penalize you for subbing in terms of f2p gaming.
I dont think they will do that. I am pretty sure, they are aware that a subbed lvl 1-20 will be able to gain way more things (enchants, gear etc.) and play with it after unsubbing than f2ps that have never been subbed. But why would they care about that? They dont care about a veteran f2p being able to get enchants, guildlooms and what not. Thats what you earned while you have been subbed.
My bet is, they let you play every toon <21. On a lapsed account you will have all the same limits than every f2p (beside guild joining). If you got more gold, you will just be able to use 10g on it etc.
I just hope they wont automatically turn xp on a 19 back on when your sub ended. That would give f2ps the possibilty to play in a more competetive bracket..
Edit: another post from Bashiok
https://mobile.twitter.com/Bashiok/status/555857040422105088