They say this is evidence that they were capable of sailing out of sight of land... but is it?
It seems just as likely to me that this ship was blown out of reach of the coast by a storm upon which it sank in deep water. If more ships from the same era are found in deep water between known ports that might be evidence of astronomical navigation but a single ship does not convince me in that respect.
This was my reaction as well - every article I've read seems to call out "changes the way we think they navigated because out of sight of land", but this may be a perverse form of survivor bias (ships tried this and sank), or as you point out, it could have been blown off course where it then sank.