Documentation for "guest only" says, 'This parameter will have no effect if guest ok is not set for the service.'

However, using 4.8.0 and the following configuration, guest access works fine:
[global]
map to guest = Bad User
vfs objects = catia fruit streams_xattr
log file = /media/ext4tba/tmp/log/samba/%m
log level = 2

# Time Machine
fruit:time machine = yes
fruit:veto_appledouble = no
fruit:encoding = native
fruit:metadata = stream

# Security
server min protocol = SMB2

[Time Mashini]
path = /media/ext4tba
guest only = yes
writeable = yes

Warwick Bruce Chapman | +27 83 7797 094 | http://wa.rwick.com