When configuring an eBGP Peer without the Multihop option (this is default), the TTL option default value still is
to use the system-default TTL of 64.
I think this is confusing, and it is not what other products do by default. When not using Multihop, the TTL
would better be set to 1. This ensures that only directly connected peers can talk to eachother. Only when
your enable Multihop, a higher TTL than 1 becomes useful.

Any opinion from the BGP experts?

Multihop is not a TTL setting it is as stated in the wiki/manual

multihop (yes | no; Default: no)

Specifies whether the remote peer is more than one hop away.

This option affects outgoing nexthop selection as described in RFC 4271 (for EBGP only, excluding EBGP peers local to the confederation).

It also affects:

whether to accept connections from peers that are not in the same network (the remote address of the connection is used for this check);
whether to accept incoming routes with NEXT_HOP attribute that is not in the same network as the address used to establish the connection;
the target-scope of the routes installed from this peer; routes from multi-hop or IBGP peers resolve their nexthops through IGP routes by default.

Sure, but in other implementations it also sets the TTL to 1 (when not multihop).
In RouterOS this setting has somehow been split in two different ones, which is kind of confusing.

Stil you get the same behaveor with the implementation in MikroTik. But if you like to have TTL 1 you can set this in the mangel but it is no need for it.